Build dxpy source distribution and use pip to install from it

This is needed because when using dxpy's "setup.py install", the
resulting installation injects a setuptools path into sys.path before
our own path, causing us to rely on an external pkg_resources file
from setuptools, and if that is too far out of date, it's unable to
resolve newer-format dependencies (ones that use "dist-info"
directories).

In particular, this should resolve the
"pkg_resources.DistributionNotFound: requests==2.3.0" message on
CentOS.