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.

先從 setup.py 和 setuptools_scm 的相依性宣告開始，接著使用 issue 中所示的自訂 -i 鏡像，透過 pip 重現空白 virtualenv 中的安裝流程。當安裝 synapse 及其相依性不需要從該鏡像另外安裝 setuptools_scm 時，即表示完成。