Release v4.0.0

[desilinguist]

• Update version number.
• Update conda recipe.
• Fix minor issue in testing setup that prevented SKLL Conda Tester and SKLL PyPI tester from working with nose2.
• Update MANIFEST.in from including tests directory into packages.
• Update doc/contributing.rst
  • Remove entry for skll/__init__.py since it's now blank.
  • Update entry for skll/metrics.py since it now does more.
  • Fix all of the link anchors to point to the correct line numbers.

v4.0 conda and TestPyPI packages have been built, deployed, and tested.

skll-conda-tester: passing skll-pip-tester: passing

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (dc50a88) 95.30% compared to head (401b20b) 95.30%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #752 +/- ## ======================================= Coverage 95.30% 95.30% ======================================= Files 29 29 Lines 3576 3576 ======================================= Hits 3408 3408 Misses 168 168 ``` | [Impacted Files](https://app.codecov.io/gh/EducationalTestingService/skll/pull/752?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://app.codecov.io/gh/EducationalTestingService/skll/pull/752?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)` | |

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