pip install of synapse with a custom package mirror fails if setuptools_scm not already installed

when attempting to run:
venv/bin/pip install -i https://python-pypi-mirror/ synapse

on a blank virtualenv, we attempt to install the dependencies via the python-pypi-mirror, including signedjson

However, the change to setup.py to require setuptools_scm causes a failure as it attempts to download setuptools_scm directly from pypi rather than via the mirror.

This can be worked around by pip install -i https://python-pypi-mirror/ setuptools_scm in the virtualenv before installing synapse, but it would be preferred if when pip installs this as a dependency, it inherited the custom package url.

This is only a problem on new installations - existing installations will likely be fine as they already have this installed.

Commencez par setup.py et la déclaration de dépendance pour setuptools_scm, puis reproduisez l’installation dans un environnement virtuel vierge avec pip en utilisant le miroir personnalisé indiqué dans l’issue. C’est terminé lorsque l’installation de synapse et de ses dépendances ne nécessite pas d’installation séparée de setuptools_scm depuis le miroir.