GeezerVison

Since gitpython-developers#1987, test jobs from `pythonpackage.yml` appear in an
unintuitive order, and some show an extra bool matrix variable in
their names while others don't (this corresponds to `experimental`,
which was always set to some value, but was set in different ways).

This fixes that by:

- Listing all tested versions, rather than introducing some in an
  `include` key. (The `include:`-introduced jobs didn't distinguish
  between originally-present matrix variables and those that are
  introduced based on the values of the original ones.)

- Replacing `os` with `os-type`, which has only the first part of
  the value for `runs-on:` (e.g., `ubuntu`), and adding `os-ver`
  to each matrix job, defaulting it to `latest`, but using `22.04`
  for Python 3.7 on Ubuntu.

This should also naturally extend to adding 3.13, with or without
setting `continue-on-error` to temporarily work around the problems
obseved in gitpython-developers#1955, but nothing 3.13-related is done in this commit.

This goes a bit further in the direction of the preceding commit,
making CI reports/logs a bit more intuitive.

We pinned Python 3.9.16 on Cygwin CI in gitpython-developers#1814 (by requiring
3.9.16-1 as the exact version of the `python39` Cygwin package,
along with other supporting changes). We did this to solve a
problem where Python 3.9.18-1, which contained a bug that broke
GitPython CI (and various other software), would be selected.

Version 3.9.18-1 was marked back to being a "test" package shortly
after the bug was reported, and was subsequently removed altogether
from the Cygwin repositories. Because the affected package version
effectively no longer exists, and because this issue is known and
a non-"test" version still affected by it is very unlikely to be
released in the future, this pinning has been decisively
unnecessary for some time, though still not harmful.

This commit undoes the pinning, so that the `python39` package can
be installed at a higher version if one becomes available. This
serves two purposes.

- There is work under way in porting Python 3.12 to Cygwin. To test
  this with GitPython (either while it is in development or later),
  it will be useful to turn the Cygwin test job into a matrix job
  definition, generating two jobs, one for Python 3.9 and one for
  Python 3.12. Since 3.12 will probably not benefit from pinning,
  dropping pinning simplifies this.

- If the port of Python 3.12 to Cygwin is successful, it might
  lead to a solution to the but that currently keeps 3.9.18 from
  being made available for Cygwin. In that case, another 3.9.18-*
  Cygwin package would be released, which we would want to use.

Although this is uncertain, the change is a simplification, so I
think it is reasonable to do now.

Note that the pinning being undone here only affects the
distinction between different 3.9.* versions. `python39` and
`python312` are different Cygwin packages altogether, with
correspondingly different `python39-*` and `python312-*` associated
packages; this is not unpinning Python 3.9 in a way that would
cause Python 3.12 to be selected instead of it.

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