CompatHelper: bump compat for "AbstractMCMC" to "3.1"

[github-actions]

This pull request changes the compat entry for the AbstractMCMC package from 2 to 2, 3.1.

This keeps the compat entries for earlier versions.

Note: I have not tested your package with this new compat entry. It is your responsibility to make sure that your package tests pass before you merge this pull request.

[coveralls]

Pull Request Test Coverage Report for Build 838885657

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 56.198%

---

Totals
Change from base Build 786879429:	0.0%
Covered Lines:	204
Relevant Lines:	363

---

💛 - Coveralls

[codecov]

Codecov Report

Merging #25 (1fe5e1c) into master (f05c279) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #25   +/-   ##
=======================================
  Coverage   56.19%   56.19%           
=======================================
  Files           5        5           
  Lines         363      363           
=======================================
  Hits          204      204           
  Misses        159      159           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f05c279...1fe5e1c. Read the comment docs.