Drop support for Python 3.7

[hukkin]

No description provided.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (8713c9f) 100.00% compared to head (5d79f04) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #406   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           15        15           
  Lines          818       817    -1     
=========================================
- Hits           818       817    -1     

Files Changed	Coverage Δ
src/mdformat/_compat.py	100.00% <100.00%> (ø)
src/mdformat/_util.py	100.00% <100.00%> (ø)
src/mdformat/plugins.py	100.00% <100.00%> (ø)
src/mdformat/renderer/_context.py	100.00% <100.00%> (ø)

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