Drop support for Python 2.7, 3.5, and 3.6

[dalthviz]

Seems like libssas-python dropped support for Python 2.7 https://github.com/sass/libsass-python/commit/d333e3c419f48f4b2d96528cb4c83db1f02adaec (there is no release with that change for the moment but still I guess the latest release that supports Python 2.7 will be the current one 0.21.0).

Taking the opportunity, maybe removing support for others already out of date Python versions like 3.5 and 3.6 could be worty.

Also, should we release QtSASS v0.3.2 adding a constraint to libsass<=0.21.0 before proceeding with dropping the support for Python <=3.6 @ccordoba12 ?

[ccordoba12]

Also, should we release QtSASS v0.3.2 adding a constraint to libsass<=0.21.0 before proceeding with dropping the support for Python <=3.6 @ccordoba12 ?

Yeah, I think that's a good idea. Then you can remove that constrain in version 0.4.0

[dalthviz]

Note: libsass-python v0.22.0 was released - https://github.com/sass/libsass-python/releases/tag/0.22.0