chores: fix tox

[amandahla]

This PR removes the version requirement for flake8.

[github-actions]

Test coverage for 2872f93

Name                             Stmts   Miss Branch BrPart  Cover   Missing
----------------------------------------------------------------------------
src/charm.py                        52      3      4      1    93%   98-100
src/charm_state.py                  35      1      8      1    95%   21
src/constants.py                     4      0      0      0   100%
src/exceptions.py                    3      0      0      0   100%
src/github_actions_exporter.py      23      0      0      0   100%
----------------------------------------------------------------------------
TOTAL                              117      4     12      2    95%

Static code analysis report

Run started:2024-09-04 16:51:43.237643

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 587
  Total lines skipped (#nosec): 0
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 0

Run metrics:
  Total issues (by severity):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
  Total issues (by confidence):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
Files skipped (0):