fix: simplify version regex and fix rc tag handling

[auscompgeek]

Description

The hand-rolled version generation was ignoring RC tags. When removing the RC tag filter, it broke:

$ python setup.py sdist
using dev release 2026.1.1postrc
Building version 2026.1.1postrc
Traceback (most recent call last):
  [snip]
  File "/usr/lib/python3.13/site-packages/setuptools/dist.py", line 294, in __init__
    self.metadata.version = self._normalize_version(self.metadata.version)
                            ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.13/site-packages/setuptools/dist.py", line 330, in _normalize_version
    normalized = str(Version(version))
                     ~~~~~~~^^^^^^^^^
  File "/usr/lib/python3.13/site-packages/packaging/version.py", line 202, in __init__
    raise InvalidVersion(f"Invalid version: {version!r}")
packaging.version.InvalidVersion: Invalid version: '2026.1.1postrc'

This fixes that, and simplifies the regex slightly so it's clearer what on earth is happening. Some surrounding code to handle PEP 440 versioning is also cleaned up to squash a warning we get otherwise for RCs:

/usr/lib/python3.13/site-packages/setuptools/dist.py:294: InformationOnly: Normalizing '2026.1.1.rc.2' to '2026.1.1rc2'

Amp-Thread-ID: https://ampcode.com/threads/T-019bb77f-d3b6-7618-b511-4109f328102f

Meta

Merge checklist:

• Pull Request title is short, imperative summary of proposed changes
• The description documents the what and why
• If this PR changes behavior or adds a feature, user documentation is updated
• If this PR touches photon-serde, all messages have been regenerated and hashes have not changed unexpectedly
• If this PR touches configuration, this is backwards compatible with settings back to v2025.3.2
• If this PR touches pipeline settings or anything related to data exchange, the frontend typing is updated
• If this PR addresses a bug, a regression test for it is added

[roberttuttle]

@auscompgeek ... will this fix address the current inability to load the proper latest photonlibpy via pyproject.toml in robot projects (in the next few days or next release here)? (or should I open up a separate new issue about not being able to install anything beyond "photonlibpy==2026.0.1-beta" (which still was tied to 2025 of course).

[auscompgeek]

This PR only does what it says on the tin.