UPDATE: Pytz is now using a numeric naming convention (i.e. 2013.7) which fixes this bug.
The latest version of PIP only installs stable versions by default, meaning anything with a pre-release version name will be ignored. Unfortunately the Pytz naming convention looks like a beta release (2013b) when it is actually the second release of 2013.
More on this bug can be found here:
For now the workaround is to add the version when installing:
pip install pytz==2013b