Fix failures in CI ahead of first release, drop Python 3.5

[ulupo]

What does this implement/fix? Explain your changes.

• Drops support for Python 3.5 (changes in setup.py, azure-pipelines.yml and README.rst).
• Edits administrative information contained in RELEASE.rst, GOVERNANCE.rst, setup.py, CODE_AUTHORS.rst, and LICENSE.
• Adds a line to setup.cfg to avoid a pytest warning.
• Tweaks doc/conf.py to avoid a warning.
• Improves documentation and/or variable names in pyflagser/flagser.py and pyflagser/flagio.py.
• Removes all nightly steps from azure-pipelines.yml as we are unlikely to do nightlies for this project.
• Improves description and order of some steps in azure-pipelines.yml.
• Creates a conftest.py file in pyflagser/tests where a command line option for pytest named --webdl is defined. When the user passes it, flag files are downloaded and stored in a temporary folder from a remote (public) bucket. This is useful if one wishes to run pytest on installed wheels, as the .flag files needed would not typically be found in ../../flagser/test relative to the location of the test files (e.g. pyflagser/tests/test_flagio.py). The use of pytest_generate_tests replaces the use of pytest.mark.parametrize in the test files as it is a well-known issue in pytest that fixtures cannot be passed to pytest.mark.parametrize, so the original idea of defining the locations of .flag files (whether they are downloaded or not) via a fixture then passed to pytest.mark.parametrize could not be implemented.
• Creates a __main__.py and a conftest.py file in pyflagser/tests to be able to run tests on installed wheels using the command python -m pyflagser.tests --webdl --no-cov --no-coverage-upload. The reason pyflagser --pyargs pyflagser --webdl could not be made to work is that it is a well-known issue (see also here) that command-line options defined in a conftest cannot generically be found when pytest --pyargs is used.

[azure-pipelines[bot]]

Azure Pipelines successfully started running 1 pipeline(s).

[ulupo]

/azp run

[azure-pipelines[bot]]

Azure Pipelines successfully started running 1 pipeline(s).

[ulupo]

@gtauzin I believe this is ready for review.