Setuptools-scm

- drop jaraco.windows from pyproject.toml, allows for wheel builds on
python2

- include python 3.9 via the deadsnakes action
- return release_branch_semver scheme (it got dropped in a bad rebase)
- undo the devendoring of the samefile backport for python2.7 on
windows
- re-enable the building of universal wheels
- fix handling of missing git/hg on python2.7 (python 3 exceptions
where used)
- correct the tox flake8 invocation
- trigger builds on tags again

- Add `parentdir_prefix_version` to support installs from GitHub
release tarballs.
- use Coordinated Universal Time (UTC)
- switch to github actions for ci
- fix documentation for `tag_regex` and add support for single digit
versions
- document handling of enterprise distros with unsupported setuptools
versions 312
- switch to declarative metadata
- drop the internal copy of samefile and use a dependency on
jaraco.windows on legacy systems
- select git tags based on the presence of numbers instead of dots
- enable getting a version form a parent folder prefix
- add release-branch-semver version scheme
- make global configuration available to version metadata
- drop official support for python 3.4

- add `no-local-version` local scheme and improve documentation for
schemes

- fix 403: also sort out resource warnings when dealing with git file
finding

- fix 399: ensure the git file finder terminates subprocess after
reading archive