Bump development version to 6.0.1.dev0
Ensure that we preserve executable file permission bits
Fix import so exception module is accesible. With the previous import `pip` module wouldn't have any `exceptions` attribute as it's __init__ doesn't actually import it. Fixes the confusing error message from #2067.
fix typo
Restore the list of versions on an error
Warn about `>` being an exclusive operator now.

E.g.: `pip install 'prettytable>0.7'` will no longer install
`prettytable==0.7.2`, because of PEP 440.

Hopefully prevents the kind of confusion see in
https://bitbucket.org/pypa/setuptools/issue/301/101-in-requirementparse-foo-10-results

    $ pip install 'prettytable>0.7'
    Collecting prettytable>0.7
      The behavior of the `>` version specifier has changed in PEP 440. `>` is now an exclusive operator, meaning that >0.7 does not match >0.7.*.
      Perhaps you want `>=` instead of `>`?
      Could not find a version that satisfies the requirement prettytable>0.7 (from versions: )
      No distributions matching the version for prettytable>0.7