Preserve environment variables when running Popen.

[rkuska]

Popen does not inherit environment variables of current process
when run with env argument defined (and not None). We therefore
pass copy of the current environment with PYTHONPATH modified to
our needs.

See:

>>> subprocess.Popen('/usr/bin/echo $LD_LIBRARY_PATH', shell=True, env={'PYTHONPATH': '.'}, stdout=subprocess.PIPE).stdout.read()
b'\n'
>>> subprocess.Popen('/usr/bin/echo $LD_LIBRARY_PATH', shell=True, stdout=subprocess.PIPE).stdout.read()
b'/opt/rh/rh-python34/root/usr/lib64\n'

This may lead to failure when trying to install python bindings of qpid-proton.
https://bugzilla.redhat.com/show_bug.cgi?id=1312590