Add infrastructure allowing for test cases for third-party stubs (#8700)

- Move the logic for running mypy on the test cases from `tests/mypy_test.py` to a separate script, `tests/regr_test.py`.
- Add the necessary logic in order to be able to have test cases for third-party stubs.
- Move logic common to `tests/mypy_test.py` and `tests/regr_test.py` into `tests/colors.py`, and rename `tests/colors.py` to `tests/utils.py`.
- Add a new check to `tests/check_consistent.py`, to enforce the use of `# pyright: reportUnnecessaryTypeIgnoreComment=true` comments in third-party test cases. These are essential if we want to have our tests against false-negatives work with pyright.
- Update the relevant documentation to account for the new test file.
- Add a new job to the `tests.yml` GitHub workflow, to run the new test in CI.
- Add a simple proof-of-concept test case for `requests`, as a regression test for #7998.

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
Co-authored-by: Sebastian Rittau <srittau@rittau.biz>

stubs/requests/@tests/test_cases/check_post.py

@@ -0,0 +1,13 @@
+# pyright: reportUnnecessaryTypeIgnoreComment=true
+
+import requests
+
+# Regression test for #7988 (multiple files should be allowed for the "files" argument)
+# This snippet comes from the requests documentation (https://requests.readthedocs.io/en/latest/user/advanced/#post-multiple-multipart-encoded-files),
+# so should pass a type checker without error
+url = "https://httpbin.org/post"
+multiple_files = [
+    ("images", ("foo.png", open("foo.png", "rb"), "image/png")),
+    ("images", ("bar.png", open("bar.png", "rb"), "image/png")),
+]
+r = requests.post(url, files=multiple_files)