Support Python 3.9 (#689)

Test PyGMT with Python 3.9

* Use Python 3.8 in environment.yml and style checks CI
* Run Python 3.6 tests on master branch only to conserve CI resources

Author: weiji14

.github/workflows/ci_tests.yaml

@@ -25,7 +25,7 @@ jobs:
       - name: Set up Python
         uses: actions/setup-python@v2
         with:
-          python-version: 3.7
+          python-version: 3.8
 
       - name: Install packages
         run: pip install black blackdoc flake8 pylint
@@ -42,8 +42,14 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: [3.6, 3.7, 3.8]
+        python-version: [3.6, 3.7, 3.8, 3.9]
         os: [ubuntu-latest, macOS-latest, windows-latest]
+        # Run Python 3.6 tests on master branch only to conserve CI resources
+        isMaster:
+          - ${{ contains(github.ref, 'master') }}
+        exclude:
+          - isMaster: false  # i.e. on Pull Requests
+            python-version: 3.6
     # environmental variables used in coverage
     env:
       OS: ${{ matrix.os }}

.github/workflows/ci_tests_dev.yaml

@@ -19,7 +19,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: [3.8]
+        python-version: [3.9]
         os: [ubuntu-20.04, macOS-10.15]
         gmt_git_ref: [master]
     env:

.github/workflows/publish-to-pypi.yml

@@ -30,7 +30,7 @@ jobs:
     - name: Set up Python
       uses: actions/setup-python@v2
       with:
-        python-version: 3.8
+        python-version: 3.9
 
     - name: Install dependencies
       run: python -m pip install setuptools wheel

doc/install.rst

@@ -138,4 +138,3 @@ e.g. in a terminal run::
     export GMT_LIBRARY_PATH=$HOME/anaconda3/envs/pygmt/lib
     # Windows
     set "GMT_LIBRARY_PATH=C:\Users\USERNAME\Anaconda3\envs\pygmt\Library\bin\"
-

environment.yml

@@ -3,7 +3,7 @@ channels:
     - conda-forge
     - defaults
 dependencies:
-    - python=3.7
+    - python=3.8
     - pip
     - gmt=6.1.1
     - numpy

setup.py

@@ -38,6 +38,7 @@
     "Programming Language :: Python :: 3.6",
     "Programming Language :: Python :: 3.7",
     "Programming Language :: Python :: 3.8",
+    "Programming Language :: Python :: 3.9",
     "License :: OSI Approved :: {}".format(LICENSE),
 ]
 PLATFORMS = "Any"