Cleanup requirements & tox config, update pylint

Cleanup requirements & tox config, update pylint

- remove useless Python packages
- make the purpose of each requirements.txt file explicit
- don't overlap between requirements.txt files
- fix requirements so that the "new" pip dependency resolver (enabled by
  default since pip 20.3) work.  Consequently, it is not necessary to pin
  pip version to 20.2.4 and we can drop tox-pip-version
- remove unused tox environments
- skip the installation of nfvbench in the tox environments where this
  is not necessary
- update pylint 2.10.2 => 2.15.9 (that was necessary to fix some
  requirement conflicts):
  - disable some of new checkers:
    consider-using-f-string,consider-using-generator,unnecessary-lambda-assignment,implicit-str-concat,global-variable-not-assigned
  - remove many obsolete checkers from the disable list
  - apply a simple refactor suggestion in nfvbench code
- upgrade to Python 3.8 some remaining references to Python 3.6

Change-Id: I55c0fb144b9199681962f396582590aba89ed02c
Signed-off-by: Gwenael Lambrouin <gwenael.lambrouin@orange.com>