remove six and python 2 compatibility + warning fix

.gitlab-ci.yml

doc/install.rst

ltfatpy/comp/assert_sigreshape_pre.py

ltfatpy/fourier/middlepad.py

ltfatpy/fourier/pherm.py

ltfatpy/fourier/psech.py

ltfatpy/gabor/dgtlength.py

ltfatpy/gabor/gabdual.py

ltfatpy/gabor/idgt.py

ltfatpy/gabor/phaselock.py

ltfatpy/gabor/phaseunlock.py

ltfatpy/signals/greasy.py

ltfatpy/signals/gspi.py

ltfatpy/signals/linus.py

ltfatpy/sigproc/firwin.py

ltfatpy/sigproc/largestn.py

ltfatpy/tests/gabor/test_gabphasegrad.py

ltfatpy/tests/signals/test_greasy.py

setup.py