CMake: drop possible use of PyROOT

[andresailer]

We no longer test with ROOT 6.22, and in 6.32 PyROOT is coming back but subtly fails on macOS

BEGINRELEASENOTES

• CMake: drop possible use of ROOT::PyROOT target, always use ROOT::ROOTTPython
• Python: remove the use of (dd)six, fix issues pointed out by new version of flake8

ENDRELEASENOTES

Accidentally picking up PyROOT leads to

ERROR: failed to load libDDG4Python.1.28.dylib: dlopen(libDDG4Python.1.28.dylib, 0x0005): symbol not found in flat namespace '__ZN9TPyReturnD1Ev'
...
ERROR: failed to load libDDPythonPlugins.1.28.dylib: dlopen(libDDPythonPlugins.1.28.dylib, 0x0005): symbol not found in flat namespace '__ZN7TPython4EvalEPKc'

[andresailer]

There is a new version of flake8 in the nightly stack giving errors in python-lint

[github-actions[bot]]

Test Results

   14 files     14 suites   6h 42m 23s :stopwatch:   363 tests   362 :white_check_mark: 0 :zzz: 1 :x: 2 496 runs  2 495 :white_check_mark: 0 :zzz: 1 :x:

For more details on these failures, see this check.

Results for commit cf2cd904.