Avoid DeprecationWarning from urllib3

[bhrutledge]

When running tests, I've started to see:

=============================== warnings summary ===============================
tests/test_repository.py: 44 warnings
  /Users/brian/Code/twine/venv/lib/python3.6/site-packages/urllib3/util/retry.py:255: DeprecationWarning: Using 'method_whitelist' with Retry is deprecated and will be removed in v2.0. Use 'allowed_methods' instead
    DeprecationWarning,

-- Docs: https://docs.pytest.org/en/stable/warnings.html

For details, see:

• https://github.com/urllib3/urllib3/blob/master/CHANGES.rst#1260-2020-11-10
• https://urllib3.readthedocs.io/en/1.26.x/reference/urllib3.util.html#urllib3.util.Retry

This is an attempt to silence that warning, while remaining flexible on the version of urllib3. I'm guessing there are other (possibly preferable) approaches. For example, we could require urllib3<2, and suppress the warning.

[sigmavirus24]

Please let's not check the version like this. Why not just do something simple like:

try:
    retry = urllib3.Retry(allowed_methods=["GET"], **retry_kwargs)
except TypeError:
    # NOTE remove once urllib3 1.26 is the minimum version
    retry = urllib3.Retry(method_whitelist=["GET"], **retry_kwargs)

[bhrutledge]

🤦‍♂️ This is much better, thanks. Done.

[bhrutledge]

I tested this locally, via b07f7db. But -e py36 isn't run by the GitHub Action; instead, it runs -e py to use the job's matrix.python version:

twine/.github/workflows/main.yml

Lines 40 to 52 in fd57198

	matrix:
	python: [3.6, 3.7, 3.8, 3.9]
	platform: [ubuntu-latest, macos-latest, windows-latest]
	runs-on: ${{ matrix.platform }}
	steps:
	- uses: actions/checkout@v2
	- uses: actions/setup-python@v2
	with:
	python-version: ${{ matrix.python }}
	- name: Install dependencies
	run: python -m pip install tox
	- name: Run tests
	run: python -m tox -e py -- --cov-report xml

With that in mind, this felt like a reasonable time to use no cover.