I propose we finally drop support for Python versions <= 3.6.

The argument for this is that now all manylinux piparch images that support Python <= 3.6 have been deprecated (as seen in https://github.com/pypa/manylinux), so supporting these is becoming untenable. This includes manylinux1 and manylinux2010, where the remaining non-deprecated and non-EOL manylinux images (manylinux2014 and manylinux_x_y) only support from Python 3.7 and above.