DOC: update testing README

tests/README.md

@@ -22,42 +22,35 @@ reveal_type(x)   # E: <type name>
 Right now, the error messages and types are must be **contained within
 corresponding mypy message**.
 
-Test files that end in `_py3.py` will only be type checked against Python 3.
-All other test files must be valid in both Python 2 and Python 3.
-
 ## Running the tests
 
 To setup your test environment, cd into the root of the repo and run:
 
 
 ```
 pip install -r test-requirements.txt
-pip install .
 ```
 
-Note that due to how mypy reads type information in PEP 561 packages, you'll 
-need to re-run the `pip install .` command each time you change the stubs.
-
-We use `py.test` to orchestrate our tests. You can just run:
+To run the tests, do
 
 ```
-py.test
+python runtests.py
 ```
 
-to run the entire test suite. To run `mypy` on a specific file (which
-can be useful for debugging), you can also run:
+from the repo root. To run `mypy` on a specific file (which can be
+useful for debugging), you can also run:
 
 ```
+pip install .  # Make sure stubs are installed; runtests does this for you
 mypy <file_path>
 ```
 
-Note that it is assumed that all of these commands target the same 
+Note that it is assumed that all of these commands target the same
 underlying Python interpreter. To ensure you're using the intended version of
 Python you can use `python -m` versions of these commands instead:
 
 ```
 python -m pip install -r test-requirements.txt
 python -m pip install .
-python -m pytest
 python -m mypy <file_path>
 ```