[FEATURE] Support python 3.6

[MichalChromcak]

Is your feature request related to a problem? Please describe.
There seems to be no problem in using 3.6, so why not to keep it

Describe the solution you'd like
Change setup.cfg from python requires >=3.7 to >=3.6

Describe alternatives you've considered
Do not support

Additional context
Adds compatibility with sktime