Use importlib.metadata in docs build

[A5rocks]

This also makes RTD install trio instead of depending on import trio working. IMO this is more correct.

Supersedes #3190.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00000%. Comparing base (ed2a7d3) to head (ad2d101).
Report is 242 commits behind head on main.

Additional details and impacted files

@@                Coverage Diff                @@
##                 main        #3194     +/-   ##
=================================================
  Coverage   100.00000%   100.00000%             
=================================================
  Files             124          124             
  Lines           18460        19619   +1159     
  Branches         1216         1483    +267     
=================================================
+ Hits            18460        19619   +1159     

see 4 files with indirect coverage changes

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