chore: update GitHub branch references to use HEAD (#105)

Author: thinkingserious

CONTRIBUTING.md

@@ -61,7 +61,7 @@ cd smtpapi-python
 
 ##### Execute: #####
 
-See the [examples folder](https://github.com/sendgrid/smtpapi-python/tree/master/examples) to get started quickly.
+See the [examples folder](examples) to get started quickly.
 
 <a name="understanding-the-codebase"></a>
 ## Understanding the Code Base
@@ -83,9 +83,9 @@ Builds the SMTPAPI header.
 
 All PRs require passing tests before the PR will be reviewed.
 
-All test files are in the [`test`](https://github.com/sendgrid/smtpapi-python/tree/master/test) directory.
+All test files are in the [`test`](test) directory.
 
-For the purposes of contributing to this repo, please update the [`__init__.py`](https://github.com/sendgrid/smtpapi-python/blob/master/test/__init__.py) file with unit tests as you modify the code.
+For the purposes of contributing to this repo, please update the [`__init__.py`](test/__init__.py) file with unit tests as you modify the code.
 
 ```bash
 python setup.py install

Dockerfile

@@ -1,6 +1,6 @@
 FROM ubuntu:xenial
 ENV PYTHON_VERSIONS='python2.6 python2.7 python3.4 python3.5 python3.6' \
-    OAI_SPEC_URL="https://raw.githubusercontent.com/sendgrid/sendgrid-oai/master/oai_stoplight.json"
+    OAI_SPEC_URL="https://raw.githubusercontent.com/sendgrid/sendgrid-oai/HEAD/oai_stoplight.json"
 
 # install testing versions of python, including old versions, from deadsnakes
 RUN set -x \
@@ -17,7 +17,7 @@ RUN set -x \
 WORKDIR /root
 
 # install Prism
-ADD https://raw.githubusercontent.com/stoplightio/prism/master/install.sh install.sh
+ADD https://raw.githubusercontent.com/stoplightio/prism/HEAD/install.sh install.sh
 RUN chmod +x ./install.sh && \
     ./install.sh && \
     rm ./install.sh

FIRST-TIMERS.md

@@ -37,15 +37,15 @@
  6. __Locally merge (or rebase)__ the upstream development branch into your topic branch:
 
    ```    
-    git pull [--rebase] upstream master
+    git pull [--rebase] upstream main
    ``` 
 
  7. __Push__ your topic branch up to your fork:
    ```    
    git push origin <topic-branch-name>
    ```
  8. __[Open a Pull Request](https://help.github.com/articles/creating-a-pull-request/#changing-the-branch-range-and-destination-repository/)__
-    with a clear title and description against the `master` branch. All tests must be passing before we will review the PR.
+    with a clear title and description against the `main` branch. All tests must be passing before we will review the PR.
 
  ### Important notice
  Before creating a pull request, make sure that you respect the repository's constraints regarding contributions. You can find them in the [CONTRIBUTING.md](./CONTRIBUTING.md) file.

README.rst

@@ -116,24 +116,24 @@ License
 `The MIT License (MIT)`_
 
 .. _SendGrid documentation: https://sendgrid.com/docs/API_Reference/SMTP_API/index.html
-.. _CHANGELOG: https://github.com/sendgrid/smtpapi-python/blob/master/CHANGELOG.md
+.. _CHANGELOG: https://github.com/sendgrid/smtpapi-python/blob/HEAD/CHANGELOG.md
 .. _free level: https://sendgrid.com/free?source=sendgrid-python
 .. _SENDGRID_API_KEY: https://app.sendgrid.com/settings/api_keys
-.. _Example Code: https://github.com/sendgrid/smtpapi-python/tree/master/examples
+.. _Example Code: https://github.com/sendgrid/smtpapi-python/tree/HEAD/examples
 .. _milestones: https://github.com/sendgrid/smtpapi-python/milestones
-.. _CONTRIBUTING: https://github.com/sendgrid/smtpapi-python/blob/master/CONTRIBUTING.md
-.. _Feature Request: https://github.com/sendgrid/smtpapi-python/blob/master/CONTRIBUTING.md#feature-request
-.. _Bug Reports: https://github.com/sendgrid/smtpapi-python/blob/master/CONTRIBUTING.md#submit-a-bug-report
-.. _Improvements to the Codebase: https://github.com/sendgrid/smtpapi-python/blob/master/CONTRIBUTING.md#improvements-to-the-codebase
-.. _The MIT License (MIT): https://github.com/sendgrid/smtpapi-python/blob/master/LICENSE.md
+.. _CONTRIBUTING: https://github.com/sendgrid/smtpapi-python/blob/HEAD/CONTRIBUTING.md
+.. _Feature Request: https://github.com/sendgrid/smtpapi-python/blob/HEAD/CONTRIBUTING.md#feature-request
+.. _Bug Reports: https://github.com/sendgrid/smtpapi-python/blob/HEAD/CONTRIBUTING.md#submit-a-bug-report
+.. _Improvements to the Codebase: https://github.com/sendgrid/smtpapi-python/blob/HEAD/CONTRIBUTING.md#improvements-to-the-codebase
+.. _The MIT License (MIT): https://github.com/sendgrid/smtpapi-python/blob/HEAD/LICENSE.md
 
-.. |Travis Badge| image:: https://travis-ci.org/sendgrid/smtpapi-python.svg?branch=master
+.. |Travis Badge| image:: https://travis-ci.org/sendgrid/smtpapi-python.svg?branch=main
    :target: https://travis-ci.org/sendgrid/smtpapi-python
 .. |Email Notifications Badge| image:: https://dx.sendgrid.com/badge/python
    :target: https://dx.sendgrid.com/newsletter/python
 .. |Twitter Follow| image:: https://img.shields.io/twitter/follow/sendgrid.svg?style=social&label=Follow
    :target: https://twitter.com/sendgrid
-.. |Codecov branch| image:: https://img.shields.io/codecov/c/github/sendgrid/smtpapi-python/master.svg?style=flat-square&label=Codecov+Coverage
+.. |Codecov branch| image:: https://img.shields.io/codecov/c/github/sendgrid/smtpapi-python/main.svg?style=flat-square&label=Codecov+Coverage
    :target: https://codecov.io/gh/sendgrid/smtpapi-python
 .. |Python Versions| image:: https://img.shields.io/pypi/pyversions/smtpapi.svg
    :target: https://pypi.org/project/smtpapi/
@@ -142,4 +142,4 @@ License
 .. |GitHub contributors| image:: https://img.shields.io/github/contributors/sendgrid/smtpapi-python.svg
    :target: https://github.com/sendgrid/smtpapi-python/graphs/contributors
 .. |MIT Licensed| image:: https://img.shields.io/badge/license-MIT-blue.svg
-   :target: https://github.com/sendgrid/smtpapi-python/blob/master/LICENSE.md
+   :target: https://github.com/sendgrid/smtpapi-python/blob/HEAD/LICENSE.md