Python: matplotlib needs bz2 package

[dbrnz]

Matplotlib now depends on bz2, which isn't available under Windows.

We need to:

1. Check if bzip2 is available at Python build time.
2. If its not, build bzip2 as part of building Python.
3. Update our matplotlib release to remove conditional import of bz2.

[dbrnz]

Matplotlib 3.0 only imports bz2 if it has a file to open so the conditional import referred to above has been removed.