During the migration the automated script to update the copyright
headers accidentally got rid of some of the existing copyright lines.
Reinstate them.
* Fix incorrect argument in test case
* Add copyright header
* Docstring and __all__
* Exclude dev depenencies
* Use changelog from #12088
* Include version in error messages
This will hopefully distinguish between the version of the source code
and the version of the distribution package that is installed.
* Linter script is your friend
* Pull runtime dep checks into their own module
* Reimplement `check_requirements` using `importlib`
I've tried to make this clearer. We start by working out which of
Synapse's requirements we need to be installed here and now. I was
surprised that there wasn't an easier way to see which packages were
installed by a given extra.
I've pulled out the error messages into functions that deal with "is
this for an extra or not". And I've rearranged the loop over two
different sets of requirements into one loop with a "must be instaled"
flag.
I hope you agree that this is clearer.
* Test cases
