Python 3.6+ support

[svandragt]

Looking at the dependencies, this should support Python 3.6+. That would mean it would run on a raspberry pi without having to install a newer python, which is very slow due to restricted CPU resources.

[svandragt]

Was unable to use 3.6 on Raspberry PI due to the toolchain installed by the system python causing ast27/Include/pgenheaders.h:8:10: fatal error: Python.h: No such file or directory error, described in further detail in python/typed_ast#97 (comment)

Full error:

      arm-linux-gnueabihf-gcc -pthread -DNDEBUG -g -fwrapv -O2 -Wall -g -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -fPIC -Iast27/Include -I/home/pi/.cache/pypoetry/virtualenvs/twelete-b4-XDqo6-py3.7/include -I/usr/include/python3.7m -c ast27/Parser/acceler.c -o build/temp.linux-armv7l-3.7/ast27/Parser/acceler.o
      In file included from ast27/Parser/acceler.c:13:
      ast27/Parser/../Include/pgenheaders.h:8:10: fatal error: Python.h: No such file or directory
       #include "Python.h"
                ^~~~~~~~~~
      compilation terminated.
      error: command 'arm-linux-gnueabihf-gcc' failed with exit status 1