Revert c77f52aaa and c20a48f89 (do not define __version__ ourselves)

c77f52aaa aimed at making a __version__ attribute available inside the
netsnmpagent module by using pkg_resources to retrieve our version number.
c20a48f89 consequently added a RPM spec file dependency on python-setuptools.

This was clearly wrong, not only because it made running the examples from
within a git clone or a source distribution impossible until "make install"
got executed. It is just not the netsnmpagent module's job to have any
knowledge about its (externally assigned) version number. Instead of using
pkg_resources ourselves, production quality agents that use our module can
(and should) use pkg_resources if they require a specific python-netsnmpagent
version. After all, they need a installed copy of python-netsnmpagent anyway,
so pkg_resources DOES know its version. This could look like this:

[...]

Finally, the example agents in the source distribution are always designed
for the corrosponding version of the module, so no need to express explicit
version requirements here that are implicit.