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.

Bắt đầu với setup.py và khai báo dependency cho setuptools_scm, sau đó tái hiện việc cài đặt trong một virtualenv trống bằng pip với mirror tùy chỉnh được nêu trong issue. Hoàn tất khi việc cài đặt synapse và các dependency của nó không yêu cầu cài đặt riêng setuptools_scm từ mirror.