upgrade: improve documentation of matchrevlog() It was not clear what selected for cloning meant. I updated with an extra line of description.

automation: support running against Python 3.9 We just added CPython 3.9.0 to the automation environment. Let's support actions targeting it. One side-effect of this change is that we'll produce Windows wheels for Python 3.9 by default. This is desirable. Differential Revision: https://phab.mercurial-scm.org/D9189

automation: upgrade packages in Linux environment It's been a while since we've ran `pip-compile -U`. Let's do that. When I ran this command, black was upgraded and newer versions of black complains about formatting differences. I didn't feel like shaving a yak, so I pinned the current black version to avoid churn. I manually ran tests against all major Python versions and I'm fairly certain no new test failures were introduced. Differential Revision: https://phab.mercurial-scm.org/D9188

packaging: upgrade packages in Windows environment I ran `pip-compile -U` to update all packages to latest versions. I did this so the Windows environment is more modern. dulwich 0.20 dropped support for Python 2.7. Rather than add yet another requirements.txt file, I decided to just pin at an older release to minimize complexity. Differential Revision: https://phab.mercurial-scm.org/D9187

contrib: install Python 3.9.0 We update the Windows and Linux environments to install CPython 3.9.0, which was released a few days ago. Differential Revision: https://phab.mercurial-scm.org/D9186

contrib: stop installing Python 3.5 and 3.6 in Windows environment We're only publishing Python 3.7+ wheels and other builds on Windows. Python 3.5 and 3.6 are not supported on this platform. Let's stop installing them in the development environment. Differential Revision: https://phab.mercurial-scm.org/D9185