Release v3.1.0

[desilinguist]

• Update version number and copyright years.
• Conda package has been created and tested.
• TestPyPI package has been created and tested.

[codecov[bot]]

Codecov Report

Base: 96.91% // Head: 96.91% // No change to project coverage :thumbsup:

Coverage data is based on head (edec2f0) compared to base (7660981). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #718 +/- ## ======================================= Coverage 96.91% 96.91% ======================================= Files 63 63 Lines 9292 9292 ======================================= Hits 9005 9005 Misses 287 287 ``` | [Impacted Files](https://codecov.io/gh/EducationalTestingService/skll/pull/718?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EducationalTestingService) | Coverage Δ | | |---|---|---| | [skll/version.py](https://codecov.io/gh/EducationalTestingService/skll/pull/718/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EducationalTestingService#diff-c2tsbC92ZXJzaW9uLnB5) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EducationalTestingService). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=EducationalTestingService)

:umbrella: View full report at Codecov.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.