Add type annotations to _depreciate.py

[CoolCat467]

This PR adds type annotations to _depreciate.py.

With mypy, everything is good except for one error in deprecated_alias at the moment that I'm not sure how to handle.

I would love feedback and any comments on how this could be improved to be more accurate.

[codecov]

Codecov Report

Merging #2722 (a796fff) into master (cf1f3c7) will decrease coverage by 7.41%.
The diff coverage is 92.85%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2722      +/-   ##
==========================================
- Coverage   98.89%   91.48%   -7.41%     
==========================================
  Files         113      113              
  Lines       16600    16612      +12     
  Branches     3014     2663     -351     
==========================================
- Hits        16417    15198    -1219     
- Misses        126     1269    +1143     
- Partials       57      145      +88     

Files Changed	Coverage Δ
trio/_deprecate.py	97.64% <92.85%> (-2.36%)	⬇️

... and 45 files with indirect coverage changes

[TeamSpen210]

Hmm, this is a little awkward - I just opened a PR for this same thing (#2719) yesterday.

[jakkdl]

Hmm, this is a little awkward - I just opened a PR for this same thing (#2719) yesterday.

I vote for closing this one and focusing on #2719, @CoolCat467 would be lovely if you ran a diff against your changes and reviewed it.

Feel free to reopen if you disagree, but with the PR list having 9 open typing PRs I think it good to thin it out a bit.