Fix configparser import for python2

[marsam]

The latest release breaks scrapy on python2
Closes #3889

[kmike]

wow, I wonder why aren't tests detecting this - Python 2.7 tests pass on CI

[elacuesta]

Well, this is on me, sorry ✋ #3877
In my defense, the Travis env uses Python 2.7.14, and definitely has the lowercase version available.

Locally I have 2.7.15+ and they are different modules AFAICT

Python 2.7.15+ (default, Nov 27 2018, 23:36:35) 
[GCC 7.3.0] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import configparser
>>> import ConfigParser
>>> configparser
<module 'configparser' from '/home/eugenio/.local/lib/python2.7/site-packages/configparser.pyc'>
>>> ConfigParser
<module 'ConfigParser' from '/usr/lib/python2.7/ConfigParser.pyc'>
>>>

I couldn't find anything in the 2.7.14 changelog (which includes everything up to 2.7.14) that indicates an early (before 3.0) renaming. However, I did found https://pypi.org/project/configparser/, which is indeed what my local 2.7.15 runtime is using and I suspect is also what Travis uses.

$ head /home/eugenio/.local/lib/python2.7/site-packages/configparser.py
#!/usr/bin/env python
# -*- coding: utf-8 -*-

"""Convenience module importing everything from backports.configparser."""

[kmike]

In https://github.com/scrapy/scrapy/pull/3877/files#diff-ab96eed25d6502f751749c9b001f7393 SafeConfigParser call was changed to ConfigParser, which seems to be "slightly backwards incompatible"; this PR fixes the problem with Python 2, but has the same issue. I wonder if we should revert the change in this file instead.

Initially I was thinking it may be also related to case sensitivity of file system, but lower-case configparser backport makes sense. Maybe some other code brings this library as a dependency. We shouldn't rely on it.

[elacuesta]

In py3 (3.2 to be precise, but that's not supported here anymore) SafeConfigParser was renamed to ConfigParser (source) so there is no change there.
However, I must admit that it might have been bad move in my behalf to make the change in py2, although I suspect it shouldn't make an actual difference.

@kmike I'm not sure exactly what's the object that the six.moves module ends up using (and I'm too lazy to look it up 😛 ), what do you think about the following? It's more explicit IMHO.

if six.PY2:
    from ConfigParser import SafeConfigParser as ConfigParser
else:
    from configparser import ConfigParser

[kmike]

@elacuesta your proposed fix looks good to me.

[elacuesta]

Ok, I'll open a quick PR just now, just in case we plan to release a bugfix version.

[elacuesta]

@kmike Done, please see #3896

[marsam]

addressed by #3896