Use twine to check generated package and readme

[francoisfreitag]

The setup.py check command is deprecated. Use twine check instead.
Rename the tox environment to packaging, it now checks the package.

[codecov]

Codecov Report

Merging #1468 (29605f8) into main (7f2ad72) will decrease coverage by 0.16%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1468      +/-   ##
==========================================
- Coverage   86.47%   86.31%   -0.17%     
==========================================
  Files          35       35              
  Lines        1849     1849              
  Branches      258      258              
==========================================
- Hits         1599     1596       -3     
- Misses        178      181       +3     
  Partials       72       72              

Impacted Files	Coverage Δ
debug_toolbar/panels/profiling.py	86.60% <0.00%> (-2.68%)	⬇️

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[matthiask]

Thanks!