* port check-readme to twine check before dist upload
* port self install tests to github
* artifact downloads

This is a scheme that assumes that the upcoming release for most
branches is a minor release, and that only for release branches
(those whose branch names parse as a version number according to current
configuration) is the upcoming version a patch release.

Better document the differences between the schemes in README.rst

Generate the date for today in UTC

Mercurial is broken (read: crashing with bus error) on SPARC.
Nevertheless, many packages need setuptools_scm even if we never ever
use Mercurial checkouts.  This patch makes it possible to run the test
suite successfully when Mercurial is not available on the system.

Allow getting the version from the parent directory suffix.

port testsuite to pathlib, and try to resolve windows short paths

trace git toplevel mismatch (observed on win32 name shortening)

exclude python2 on windows

remove the appveyor config, its broken

drop most travis envs that we have in github workflows now

add actions for creating egg artifacts/wheels/sdists

WIP: experiment with act

WIP: fix workflow typo

WIP: add missed runs-on

fix typoed excludes

fixup: upload actuall wheel/sdist in the build step

style fixes for the actions config

Add a `parentdir_prefix_version` config parameter which allows lets
setuptools_scm get a version string from the name of the parent
directory: if `parentdir_prefix_version = "foo-"` and the parent
directory is named `foo-v12.34`, then the version is 12.34.  This is
only active if the parameter is set (it defaults to unset, i.e. the
feature is opt-in), if getting the version from the SCM failed, *and*
there's no metadata already present.  However, this has higher priority
than `fallback_version` (which remains the last-resort fallback).

This feature is intended to support the tarballs that GitHub
automatically creates for all releases, which have no metadata but
follow the correct naming scheme for the feature to work.

This feature is directly inspired from versioneer's `parentdir_prefix`
setting.

The default `git describe` command had an undocumented `--match *.*` in it,
intended to skip over tags that may represent human-readable checkpoints.

Unfortunately this means it would be ignoring single-component versions.

Instead look for presence of digits in the tag, it's still a heuristic, but
it should cover all previous cases, and fix single component versions.