do not require running tests locally (#5356)

CONTRIBUTING.md

@@ -9,7 +9,10 @@ are important to the project's success.
 ## The contribution process at a glance
 
 1. Read the [README.md file](README.md).
-2. Set up your environment to be able to [run all tests](README.md#running-the-tests).  They should pass.
+2. If you want, set up your environment to be able to [run tests](README.md#running-the-tests).
+    This can be useful for big pull requests or fixing specific errors, but
+    usually is not needed, because the tests run automatically on GitHub Actions
+    for all pull requests.
 3. [Prepare your changes](#preparing-changes):
     * Small fixes and additions can be submitted directly as pull requests,
       but [contact us](#discussion) before starting significant work.

README.md

@@ -100,8 +100,10 @@ If you have questions related to contributing, drop by the [typing Gitter](https
 
 ## Running the tests
 
-The tests are automatically run on every PR and push to
-the repo.
+The tests are automatically run on every PR and push to the repo.
+Therefore you don't need to run them locally, unless you want to run
+them before making a pull request or you want to debug some problem without
+creating several small commits.
 
 There are several tests:
 - `tests/mypy_test.py`