remove unnecessary dependencies

[warner]

Let's remove characteristic right now, it's a throwback to some old pyopenssl thing, and the only place it exists at all in our depenency graph is in tahoe's _auto_deps.py.

Let's remove the transitive dependencies: things we claim to depend upon but never import:

• pyopenssl
• service-identity
• pyasn1-modules
• pyasn1

Let's do a global s/simplejson/json/ and stop depending on the simplejson module. The json in stdlib should be enough. (This is a leftover from the days of python2.5(?) that didn't have stdlib json, but now we require py2.7).

We need to retain twisted[tls] instead of just twisted because pip lacks a resolver.

Not a part of this ticket:

• #2098 : get zfec to remove dependency on pyutil/zbase32

[warner]

I'm working on this. The simplejson change is a bit involved (a couple of dozen files, and some use the json symbol as method or variable names already), but I'm almost done.

[Brian Warner <warner@…>]

In c1ed556/trunk:

Merge PR405: use stdlib "json" instead of simplejson

refs ticket:2766

[exarkun]

​https://github.com/tahoe-lafs/tahoe-lafs/pull/642

[GitHub <noreply@…>]

In 6d14a2d/trunk:

Merge pull request #642 from tahoe-lafs/2766.remove-unnecessary-dependencies

Remove declaration of some unnecessary dependencies.

Fixes: ticket:2766