Unpin flake8 version from requirements-dev.txt

Upstream flake8 lint test is executed with the latest available version
in pip, but the requirements-dev.txt had a pinned version, making flake8
error to be found too late.

Signed-off-by: Rafael Guterres Jeffman <rjeffman@redhat.com>

requirements-dev.txt

 -r requirements-tests.txt
 ipdb==0.13.4
 pre-commit==2.20.0
-flake8==7.0.0
+flake8
 flake8-bugbear
 pylint>=3.2
 wrapt==1.14.1