* Don't test forward compatibility in qpy compat tests

The qpy compatibility tests are designed to test backwards
compatibility. They generate QPY files using all historical releases
of Qiskit with QPY support and test that all proposed changes are
able to load the qpy files. There was an underlying assumption to how
the tests were written that the version under test is always newer
than all the historical releases. This has been true in the past because
the release model for qiskit mean that we were maintaining at most one
stable release at a time, and when we released a new minor version of
qiskit we retired the previous minor version release series's branch.
However, moving forward this assumption will no longer be true as we're
moving to a model where we support > 1 branch at a time. When we start
supporting > 1 release series at a time, PRs proposed to the older
stable branches will no longer be able to test all releases because QPY
only support backwards compatibility and will not be able to load QPY
files generated from newer releases. The example of this was in the
0.25.x series, there is overlap in the support of the stable/0.25 branch
and the stable/0.45 branch, and after the release of 0.45.0rc1 the tests
started to fail because you can't load QPY files generated with
0.45.0rc1 with 0.25.x.

This commit address this by adding a condition to the QPY compat scripts
to ensure that the version under test is >= the QPY file source version.
If we encounter that scenario we just skip the tests. This should
prevent this situation from occurring for future releases as we expand
our support windows.

* Fix syntax errors

* Use a Python script to compare versions

This commit adds a small python script to compare two version strings
and determine whether the version under test is newer (or the same) as
the source version. The logic for making this determination was a bit
more involved in bash, so using a python script lets us reuse the regex
built into qpy's interface to support this comparison already.

* Fix lint

* Fix return code with invalid version

This commit fixes the return code generation if an invalid comparison is
requested. We want to handle this with a 0 return code to treat it as a
skip, but the previous logic would error before we evaluated the if
statement.

Co-authored-by: Jake Lishman <jake.lishman@ibm.com>

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Co-authored-by: Jake Lishman <jake.lishman@ibm.com>
(cherry picked from commit 2b5503c)