WIP: configure: Add checking python3
This is another continuation of !1989 (closed). (I'll remove "WIP" in the title after ghc-8.10 branch is cut.)
We need python3 instead of python2 for the following reasons:
- To run testsuites
- To build documentation with python3-sphinx
- To deal with python2 expiration at the end of 2019
If your python is version 2, it's treated as configuration ERROR.
See also !2016 (closed).