Remove usedevelop from tox.ini

[V02460]

Removes usedevelop=true from tox.ini, which is no longer needed because #15 added the tests to the source distribution.

Also fixes errors running tox -e py repeatedly, which was expecting a setup.py previously.

[anoadragon453]

Note that Synapse's copy of this setting has a few more words:

# As of twisted 16.4, trial tries to import the tests as a package (previously
# it loaded the files explicitly), which means they need to be on the
# pythonpath. Our sdist doesn't include the 'tests' package, so normally it
# doesn't work within the tox virtualenv.
#
# As a workaround, we tell tox to do install with 'pip -e', which just
# creates a symlink to the project directory instead of unpacking the sdist.
#
# (An alternative to this would be to set PYTHONPATH to include the project
# directory. Note two problems with this:
#
#   - if you set it via `setenv`, then it is also set during the 'install'
#     phase, which inhibits unpacking the sdist, so the virtualenv isn't
#     useful for anything else without setting PYTHONPATH similarly.
#
#   - `synapse` is also loaded from PYTHONPATH so even if you only set
#     PYTHONPATH for the test phase, we're still running the tests against
#     the working copy rather than the contents of the sdist. So frankly
#     you might as well use -e in the first place.
#
# )
usedevelop=true

Also note that Synapse includes the tests package in its source distribution, but set usedevelop=true afterwards in matrix-org/synapse#2439.

[reivilibre]

This would be inline with what happened over here: matrix-org/synapse-module-cookiecutter-template#21

It's a bit of a workaround for a Tox bug but most of us don't habitually use Tox so wouldn't notice the slowdown anyway.

(N.B. I'm not sure the comment from Synapse is the whole truth. The reason it works in this scenario is because the -m in python -m twisted.trial adds . to the PYTHONPATH. Running as purely trial doesn't work not in editable mode IIRC? Though we likely don't care as long as we know how to do what works.)

[richvdh]

again, lgtm, but needs sign-off, please

[DMRobertson]

475fc80 is signed-off.

[DMRobertson]

Not fully sure how to merge this with #19 though (sorry, not familiar with tox). Could you take a quick look @V02460?

[V02460]

Fixed the conflict, forgot you dislike rebases 🙈