PEP 440: Update regex from packaging project

[RazerM]

See https://github.com/pypa/packaging/pull/705

• Change is either:
  • [ ] To a Draft PEP
  • [ ] To an Accepted or Final PEP, with Steering Council approval
  • [x] To fix an editorial issue (markup, typo, link, header, etc)
• [x] PR title prefixed with PEP number (e.g. PEP 123: Summary of changes)

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[RazerM]

While it's true that the compiled regex in the example doesn't have the issue (because it uses $), I think it's worth updating since it lets people use VERSION_PATTERN in more cases.