MAINT: Fix imports and clean up project configuration

[raphaelvallat]

• Remove dead sys.path.insert in docs/conf.py (ran after import antropy, so had no effect)
• Remove incorrect py-modules = ["antropy"] from pyproject.toml (for single-file modules only; package discovery via packages.find was already correct)
• Add explicit testpaths and pythonpath = ["tests"] to pytest config, replacing the implicit sys.path injection that made from utils import work in test
  files
• Bump minimum pytest to 7.0 (required for pythonpath ini option)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (f28cbfc) to head (0ef23fa).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #50   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines          312       312           
  Branches        52        52           
=========================================
  Hits           312       312           

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