Sincegitpython-developers#1987, test jobs from `pythonpackage.yml` appear in anunintuitive order, and some show an extra bool matrix variable intheir 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 withoutsetting `continue-on-error` to temporarily work around the problemsobseved ingitpython-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 ingitpython-developers#1814 (by requiring3.9.16-1 as the exact version of the `python39` Cygwin package,along with other supporting changes). We did this to solve aproblem where Python 3.9.18-1, which contained a bug that brokeGitPython CI (and various other software), would be selected.Version 3.9.18-1 was marked back to being a "test" package shortlyafter the bug was reported, and was subsequently removed altogetherfrom the Cygwin repositories. Because the affected package versioneffectively no longer exists, and because this issue is known anda non-"test" version still affected by it is very unlikely to bereleased in the future, this pinning has been decisivelyunnecessary for some time, though still not harmful.This commit undoes the pinning, so that the `python39` package canbe installed at a higher version if one becomes available. Thisserves 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 Ithink it is reasonable to do now.Note that the pinning being undone here only affects thedistinction between different 3.9.* versions. `python39` and`python312` are different Cygwin packages altogether, withcorrespondingly different `python39-*` and `python312-*` associatedpackages; this is not unpinning Python 3.9 in a way that wouldcause Python 3.12 to be selected instead of it.

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