rpm: don't try to package python 3 bindings if it's not installed
rpm: don't try to package python 3 bindings if it's not installed

* Don't package python3 bindings if python3_sitelib isn't defined
* Run python tests and BuildRequire pytest and python-cffi
* BuildRequire memcached, it's needed for make check

Don't run tests for Python 3 bindings just yet, the tests (and possibly
omcache.py) require some work before we can enable them.  `get()` and
`get_multi()` return `bytes` in Python 3, but the tests expect values to be
`str`.  We can't unconditionally turn `bytes` into `str` (unicode) on Python
3 as that would break use cases that use other encodings or binary data.

ohmu/omcache,ohmu/omcache,CrowdStrike/omcache,thedrow/omcache,thedrow/omcache,CrowdStrike/omcache