[AstroPy] Python installation: erratum

[Tom Aldcroft]

Please pardon me for one more mail about the Practical Python
Workshops.  I was horrified to just discover that an extra ".. " in
the installation source RST file wiped out the *key* links to the
installation instructions for Mac, Linux, and Windows.  They have been
restored:

http://python4astronomers.github.com/installation/python_install.html#install-core-python

- Tom