Note that argcomplete is also now required.

This is optional and helps with development.  If we ever need/want to
support systems without Cython, this can be wrapped in a
try/except:pass block (and the .c files can be included in the
MANIFEST.in so that source distributions include them).

This is a pretty big simplification: the user should first use pip to
install everything in `requirements.txt`, and we no longer try to
cater to cases where Cython is missing, etc.

This was in setup.py to reduce litter in the root directory, but it's
simpler to keep it how it's supposed to be.

simplejson was there for python <= 2.6 compatibility.
It's not needed now, and installing it with pip can lead to segfaults
because pip is dumb.
(see e.g. https://github.com/simplejson/simplejson/issues/114)

We don't actually use nilmdb.utils.threadsafety.verify_proxy in the
main NilmDB code, but it's useful for finding errors.

It found an issue with __getattr__ in SerializerProxy which
(1) can't be avoided?
(2) is now commented in the code
(3) shouldn't matter in real use

__getattr__ is only called for attributes that don't exist; callers
should never purposely see the private __* symbols, so we don't need
to check for them.