build: revert all attempts to disable docs travis-ci will remain broken for nightly (3.12) testing. Trying to enable maintainer mode broke on a missing dot executable. Disable nightly since something is borked in the base config there and I don't want to waste credits on it.

#!/usr/bin/env python
from __future__ import print_function
import zlib, base64, sys
print(base64.encodestring(zlib.compress(sys.stdin.read())))