DOC/BLD: Update version support in docs and setup.py.

Fixes #4412.

Changes install.rst to explicitly reference that pandas supports 2.6,
2.7, and 3.2+. (and removes line that Python 3 support is less
well-tested). Also adds 2.6, 2.7, 3.2, and 3.3 in classifiers in
setup.py