Setuptools-scm

- fix 509: support `SETUPTOOLS_SCM_PRETEND_VERSION_FOR_${DISTRIBUTION_NAME}` for `pyproject.toml`

Breaking changes:

- fix 339: strict errors on missing scm when parsing a scm dir to
avoid false version lookups
- fix 337: if relative_to is a directory instead of a file, consider
it as direct target instead of the containing folder and print a
warning

Bugfixes:

- fix 352: add support for generally ignoring specific vcs roots
- fix 471: better error for version bump failing on complex but
accepted tag
- fix 479: raise indicative error when tags carry non-parsable
information
- Add `no-guess-dev` which does no next version guessing,
just adds `.post1.devN` in case there are new commits after the tag
- add python3.9
- enhance documentation
- consider SOURCE_DATE_EPOCH for versioning
- add a version_tuple to write_to templates
- fix 321: add support for the
`SETUPTOOLS_SCM_PRETEND_VERSION_FOR_${DISTRIBUTION_NAME}` env var to
target the pretend key
- fix 142: clearly list supported scm
- fix 213: better error message for non-zero dev numbers in tags
- fix 356: add git branch to version on describe failure

- disallow git tags without dots by default again - 449

- 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