Update for Windows platform. Resolves #18096 (#18097)

Fixes #18096

CONTRIBUTING.md

@@ -65,16 +65,16 @@ However, if you wish to do so, you can run the full test suite
 like this:
 
 ```bash
-python3 runtests.py
+python runtests.py
 ```
 
 Some useful commands for running specific tests include:
 
 ```bash
 # Use mypy to check mypy's own code
-python3 runtests.py self
+python runtests.py self
 # or equivalently:
-python3 -m mypy --config-file mypy_self_check.ini -p mypy
+python -m mypy --config-file mypy_self_check.ini -p mypy
 
 # Run a single test from the test suite
 pytest -n0 -k 'test_name'
@@ -117,7 +117,7 @@ tox -e dev --override testenv:dev.allowlist_externals+=env -- env  # inspect the
 ```
 
 If you don't already have `tox` installed, you can use a virtual environment as
-described above to install `tox` via `pip` (e.g., ``python3 -m pip install tox``).
+described above to install `tox` via `pip` (e.g., ``python -m pip install tox``).
 
 ## First time contributors