PyModules: fix versions of flake8 externals

Specify a version range when searching the flake8 externals. This is to ensure that we are not picking up an old incompatible version from LCG. Also fix the dependencies of flake8 to their versions when installing via pip to make the build reproducible.

Example output:

-- Configuring the build of package: PyModules
-- Found Python: /cvmfs/sft.cern.ch/lcg/releases/LCG_101_ATLAS_2/Python/3.9.6/x86_64-centos7-gcc11-opt/bin/python3.9 
(found suitable version "3.9.6", minimum required is "3.9") found components: Interpreter 
-- No suitable version of pyflakes (2.4.0...<2.5.0) found in LCG. Will build it as part of PyModules.
-- No suitable version of pycodestyle (2.8.0...<2.9.0) found in LCG. Will build it as part of PyModules.
-- Found mccabe version 0.6.1 in LCG

Fixes ATLINFR-4466.