[Experiment] CI Check deprecation for future releases

[oerc0122]

          Do we want to change the standard here to always deprecate as literally `vx.y.z` and add a sed/grep call to ensure there are none remaining before a release can be pushed.

Originally posted by @oerc0122 in pace-neutrons/pace-developers#31 (comment)

[ajjackson]

Slightly more detail:

• Currently the first step in the release instructions is to search for "deprecated" directives which were written with a speculative version number. E.g. say we are currently on v1.5.0:

  • a deprecation warning is added "deprecated since v1.6.0"
  • then we need to make a bugfix release v1.5.1, which includes this warning
  • then the version number in the warning should be corrected to "v1.5.1"

This rarely happens, is tedious to check and could be included in the version-number-bump automation. A couple of possible schemes are:

• unreleased deprecation warnings should have the literal version test "vx.y.z". This will be easy to grep out and set to the new version, but slightly harder to remember/communicate to developers.
• unreleased deprecation warnins should be set to any future version number e.g. "v1.6" or "99.9". A slightly smarter regex finds this and uses packaging.version.Version to identify that this is greater than our v1.5.1 release and so should be set to "v1.5.1".