pip/vendor/distlib/compat.py: Use our own implementation of match_hostname if `from ssl import match_hostname` raises an ImportError.

This solves an error on Python 3.1:

```python
(py31.venv)marca@marca-mac:~/dev/git-repos/pip$ pip --version
Traceback (most recent call last):
  File "/Users/marca/dev/git-repos/pip/py31.venv/bin/pip", line 9, in <module>
    load_entry_point('pip==1.5.dev1', 'console_scripts', 'pip')()
  File "/Users/marca/dev/git-repos/pip/py31.venv/lib/python3.1/site-packages/pkg_resources.py", line 378, in load_entry_point
    return get_distribution(dist).load_entry_point(group, name)
  File "/Users/marca/dev/git-repos/pip/py31.venv/lib/python3.1/site-packages/pkg_resources.py", line 2566, in load_entry_point
    return ep.load()
  File "/Users/marca/dev/git-repos/pip/py31.venv/lib/python3.1/site-packages/pkg_resources.py", line 2260, in load
    entry = __import__(self.module_name, globals(),globals(), ['__name__'])
  File "/Users/marca/dev/git-repos/pip/pip/__init__.py", line 11, in <module>
    from pip.util import get_installed_distributions, get_prog
  File "/Users/marca/dev/git-repos/pip/pip/util.py", line 17, in <module>
    from pip.vendor.distlib import version
  File "/Users/marca/dev/git-repos/pip/pip/vendor/distlib/version.py", line 13, in <module>
    from .compat import string_types
  File "/Users/marca/dev/git-repos/pip/pip/vendor/distlib/compat.py", line 140, in <module>
    from ssl import match_hostname, CertificateError
ImportError: cannot import name match_hostname
```

I took the existing `match_hostname` unmodified and simply use it whenever `from ssl import match_hostname` raises an ImportError.