package: read a specific Python version requirement file

This change makes pbr reads a specific requirement file tight to the
major Python version that is used. So you can add a file such as
requirements-py3.txt to have a specific requirement file for Python 3,
or requirements-py2.txt to have a specific requirement file for
Python 2.

Change-Id: I4b548e9830586f11b82539d334dd5cf4b0445a36
Do not force log verbosity level to info

Forcing the log level to info causes pbr to emit status log messages
in a way that breaks commands like "python setup.py --maintainer" and
"python setup.py --name", which are useful when scripting operations
that work on packages.

Change-Id: I6607c96e171f2f8efa89bd701018f4af6ec9c15a
Remove tox locale overrides

* tox.ini: The LANG, LANGUAGE and LC_ALL environment overrides were
introduced originally during the testr migration in an attempt to be
conservative about the possibility that locale settings in the
calling environment could cause consistency problems for test runs.
In actuality, this should be unnecessary and any place where it does
cause issues ought to be considered an actual bug. Also, having
these in the configuration actively causes older pip to have
problems with non-ASCII content in some package metadata files under
Python 3, so drop it now.

Change-Id: Ic09e1f7f90216064d550bc06c6301d5e1779b5aa
Closes-Bug: #1277495
Merge "Remove tox locale overrides"