Fix unicode encoding errors inside pip

[suutari-ai]

On Python 2, if a distribution package contains non-ASCII file names,
the Resolver failed with the following UnicodeDecodeError, because
RequirementSet was initialized with unicode paths rather than str.

Fix this by making sure that the path components used in PyPIRepository
are all filesystem encoded str.

Drop the unicode_literals import from _compat/tempfile.py, because the
strings used in there should also be str, not unicode.

Also make sure that the InstallRequirement is always constructed from
str, not unicode.

Traceback of the error looked like this:

Traceback (most recent call last):
  File ".../piptools/scripts/compile.py", line 184, in cli
    results = resolver.resolve(max_rounds=max_rounds)
  File ".../piptools/resolver.py", line 107, in resolve
    has_changed, best_matches = self._resolve_one_round()
  File ".../piptools/resolver.py", line 195, in _resolve_one_round
    for dep in self._iter_dependencies(best_match):
  File ".../piptools/resolver.py", line 274, in _iter_dependencies
    dependencies = self.repository.get_dependencies(ireq)
  File ".../piptools/repositories/pypi.py", line 145, in get_dependencies
    self._dependencies_cache[ireq] = reqset._prepare_file(self.finder, ireq)
  File ".../pip/req/req_set.py", line 620, in _prepare_file
    session=self.session, hashes=hashes)
  File ".../pip/download.py", line 821, in unpack_url
    hashes=hashes
  File ".../pip/download.py", line 663, in unpack_http_url
    unpack_file(from_path, location, content_type, link)
  File ".../pip/utils/__init__.py", line 605, in unpack_file
    untar_file(filename, location)
  File ".../pip/utils/__init__.py", line 551, in untar_file
    path = os.path.join(location, fn)
  File "/usr/local/lib/python2.7/posixpath.py", line 73, in join
    path += '/' + b
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe3 in position 66:
  ordinal not in range(128)

Fixes #564

[vphilippon]

Could you add the bugfix in the changelog? That fix deserves a mention.

For the change, LGTM

[vphilippon]

Checked out your branch, tested, it works.
This also fixes #433 (I can reproduce everytime using the --rebuild option).

[vphilippon]

I noticed your branch isn't based on master. I'll rebase your branch, add the changelog entry and submit a PR, that'll save you the trouble.

Thanks for the nice work!

[vphilippon]

"Merged" by #568
Thanks again @suutari-ai