Add extras_require to top-level setup.py to list Twisted optional dependancies

[thijs]

Setuptools provides a feature for listing optional dependancies:  extras_require. It seems like this functionality should be in trunk/twisted/python/dist.py or trunk/twisted/python/release.py or something, especially given that trunk/twisted/conch/topfiles/setup.py needs to know about the pycrypto dependency somehow too.

[glyph]

For previous discussion, see #976, which spawned this ticket.

[glyph]

Oops I meant #967.

[zooko]

See also #3238 (patch: declare that twisted requires pywin32 if it is to offer process management or iocp reactor on Windows).

[zooko]

See also  http://allmydata.org/trac/tahoe/ticket/774 (pycrypto package is required for manhole). We're going to declare that Tahoe-LAFS depends on pycrypto. But that isn't really right. Really it is that Tahoe-LAFS depends on Twisted's "manhole" feature, which depends on pycrypto. So for example, if a future release of Twisted implemented manhole without using pycrypto, then Tahoe-LAFS would not require pycrypto anymore. So the "best" way to solve this is for Tahoe-LAFS to specify that it depends on Twisted plus the "extra" of manhole, and for Twisted to declare that if you want the "extra" of manhole then you need pycrypto.

[davidsarah]

Also, twisted.conch depends on pycrypto and pyasn1.

[zooko]

Opened #4438 which emphasizes that if Twisted fails to declare an existing dependency then it is a bug, but if Twisted fails to declare a new dependency then it is a regression.