error msg if version is missing

[mathause]

Adds a clearer error message a package is missing a version in the environment file (and is not in exclude. Currently raises AttributeError: 'NoneType' object has no attribute '_version'

Traceback (most recent call last):
  File "/home/runner/work/_actions/xarray-contrib/minimum-dependency-versions/v0.1.1/minimum_versions.py", line 414, in <module>
    main()
  File "/usr/lib/python3/dist-packages/click/core.py", line 1157, in __call__
    return self.main(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/.local/lib/python3.12/site-packages/rich_click/rich_command.py", line 187, in main
    rv = self.invoke(ctx)
         ^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/click/core.py", line 1434, in invoke
    return ctx.invoke(self.callback, **ctx.params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/click/core.py", line 783, in invoke
    return __callback(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/work/_actions/xarray-contrib/minimum-dependency-versions/v0.1.1/minimum_versions.py", line 386, in main
    status = compare_versions(environments, policy_versions, policy.ignored_violations)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/work/_actions/xarray-contrib/minimum-dependency-versions/v0.1.1/minimum_versions.py", line 247, in compare_versions
    env_status = any(
                 ^^^^
  File "/home/runner/work/_actions/xarray-contrib/minimum-dependency-versions/v0.1.1/minimum_versions.py", line 250, in <genexpr>
    and spec.version > policy_versions[spec.name].version
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/.local/lib/python3.12/site-packages/rattler/version/version.py", line 544, in __lt__
    return self._version < other._version
                           ^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute '_version'

[mathause]

Not so sure about the test 🤷‍♂️

[keewis]

we might need to clean this up a bit: currently _main would call sys.exit, which is not great for testing. It also should take the contents of the files instead of file handles / paths, which avoids the need to do I/O in the tests.

Additionally, _main is not really a descriptive name. How about something like validate_environment instead?

[keewis]

I wonder if this should be done at the same time as applying exclude? I don't have the time to come up with something better right now, but assuming this isn't too urgent I will spend some time on this later this week.

[keewis]

also, does this close #28?

[keewis]

In #42 I've accidentally fixed this, and in a way that I think is much easier to understand: version = None results in empty version and date fields, plus a warning asking to pin the version. I will thus mark this as closed by #42, but will take your policy file as a new e2e test.

[mathause]

No worries! I meant to work on this but haven't so I am happy you did!