remove ImportError hack for __version__

The previous "except ImportError" hack that allowed __version__ to be imported into setup.py caught a legitimate ImportError when an environment's requests module is of an old backwards-incompatible version.

__version__ et al were moved to __about__.py
__about__.py is read and exec'd into a dict in setup.py, circumventing the need to import __version__ from rauth
__version__ et al are imported from __about__.py into __init__.py to preserve the variables' original location
information in __about__.py is deduplicated in the call to setup()