Add service_identity dependency for Twisted.

Fixes UserWarning: You do not have a working installation of the
service_identity module: 'No module named 'service_identity''.
Please install it from <https://pypi.python.org/pypi/service_identity> and
make sure all of its dependencies are satisfied.
Without the service_identity module, Twisted can perform only rudimentary
TLS client hostname verification.  Many valid certificate/hostname mappings
may be rejected.

	modified:   setup.py

setup.py

@@ -14,7 +14,8 @@
                         'requests',
                         'sqlalchemy',
                         'twisted',
-                        'python-dateutil'],
+                        'python-dateutil',
+                        'service_identity'],  # See commit notes: Needed by Twisted.
       include_package_data=True,
       scripts=['src/pdm/bin/pdm'],
      )