Proposal: Remove regress-python3-mangle.mk

Hello.
Now we generate tests for plpython3 of plpython2 tests. I think we should
write independently 2 test suite.
Why is that bad:
1. Differences between python2 and python3 more than can be solved by
regexps. And these differences are growing.
2. We must be careful to write tests, so as not to break the converter. And
we can not verify the possibility python3.
3. Because we use sed we do not tests for plpython3 under Windows. And I
have trouble with CMake too.

Yes, we will have 2 times more similar code but you need to relates to the
python2 and python3 as the different languages.

Thank you for attention.
--
Yury Zhuravlev
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company