Fix false positives in animated for PartialEmoji.from_str

[AbstractUmbra]

Summary

Fixes #9456
Test cases can be found here: https://regex101.com/r/d8WD6t/1

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[Rapptz]

I'm unsure why a positive lookahead is used, you can just put it as part of the animated group, e.g. <?(?P<animated>a:)?:?(?P<name>[A-Za-z0-9\_]+):(?P<id>[0-9]{13,20})>?. Alternatively you could do a conditional but that makes it confusing to read.

[Rapptz]

Actually a better regex might be <?(?:(?P<animated>a)?:)?(?P<name>[A-Za-z0-9\_]+):(?P<id>[0-9]{13,20})>? since this one avoids the lookahead, embeds the conditional, and makes it so ::foo:1283908120938 is not a valid input

[AbstractUmbra]

Good call! Updated that now.