fix: target python version

[stevenh]

Allow any valid python version to be set as the target version so that users don't need to install a new package version each time a new Python version is released.

Add tests for validating the target version configuration.

Clarify the meaning of the target version in the doc comment.

Fixes: #63

[hramezani]

Sorry for the delay in reviewing this 🙏

Nice fix for the set_config/ExamplesConfig drift and the py310 typo. A few things I think are needed before this is complete:

1. Validate against actual Black support, not a regex. black_mode() does BlackTargetVersion[target_version.upper()], so any well-formed-but-unsupported value still KeyErrors at format time. py315, py3100, and py99 all pass py\d{2,}$ but crash Black — and the test suite currently asserts py3100 is valid, which codifies that. Checking membership in BlackTargetVersion fixes it and auto-tracks new versions without maintenance:

   valid = {v.name.lower() for v in BlackTargetVersion}
   if self.target_version and self.target_version.lower() not in valid:
       raise ValueError(f'Invalid target_version {self.target_version!r}; supported: {", ".join(sorted(valid))}')

2. Regex edge case: $ matches before a trailing newline, so 'py37\n' passes. Moot if you switch to the membership check.
3. Keep Literal[...] | str rather than plain str — type checkers still autocomplete the known versions while accepting new ones, which keeps the discoverability the Literal gave.
4. Add a version-behavior test, not just validation. With upgrade=True, ruff's UP007 (Use X | Y) only fires at py310+, so the same snippet formats differently across versions — a parametrized test locks that in and proves py312/py313 actually flow through Black+ruff:

   union_code = 'from typing import Union\n\n\ndef foo(x: Union[int, str]):\n    pass\n'
   
   @pytest.mark.parametrize('v', ['py37', 'py38', 'py39'])
   def test_upgrade_keeps_union_pre_py310(v):
       ruff_check(CodeExample.create(union_code), ExamplesConfig(target_version=v, upgrade=True))
   
   @pytest.mark.parametrize('v', ['py310', 'py311', 'py312', 'py313'])
   def test_upgrade_rewrites_union_py310_plus(v):
       with pytest.raises(FormatError, match='UP007 .* Use `X | Y`'):
           ruff_check(CodeExample.create(union_code), ExamplesConfig(target_version=v, upgrade=True))

No pressure on turnaround — if you don't have time to make these changes, I'm happy to take care of them in a separate follow-up PR.