set proper requirements.txt

for both tests and production

requirements-swh.txt

 # Add here internal Software Heritage dependencies, one per line.
 swh.core
+swh.model

requirements-test.txt

 pytest
+requests_mock

requirements.txt

 # Add here external Python modules dependencies, one per line. Module names
 # should match https://pypi.python.org/pypi names. For the full spec or
 # dependency lines, see https://pip.readthedocs.org/en/1.1/requirements.html
+python-dateutil
+requests
 vcversioner