replace nevow with twisted.web.templates

[zooko]

Users still occasionally have problems with the nevow dependency:

​https://bugs.launchpad.net/nevow/+bug/812537

Searching for Nevow>=0.6.0
Reading http://pypi.python.org/simple/Nevow/
Reading http://divmod.org/trac/wiki/DivmodNevow
Reading http://www.divmod.org/
Reading http://divmod.org/projects/nevow
Best match: Nevow 0.10.0
Downloading https://tahoe-lafs.org/source/tahoe-lafs/deps/tahoe-dep-sdists/Nevow-0.10.0.tar.gz
Processing Nevow-0.10.0.tar.gz
Running Nevow-0.10.0/setup.py -q bdist_egg --dist-dir /tmp/easy_install-TCxz5d/Nevow-0.10.0/egg-dist-tmp-nHwooE
Traceback (most recent call last):
  File "/ffp/bin/easy_install", line 8, in <module>
    load_entry_point('setuptools==0.6c11', 'console_scripts', 'easy_install')()
  File "build/bdist.linux-armv5tejl/egg/setuptools/command/easy_install.py", line 1712, in main
  File "build/bdist.linux-armv5tejl/egg/setuptools/command/easy_install.py", line 1700, in with_ei_usage
  File "build/bdist.linux-armv5tejl/egg/setuptools/command/easy_install.py", line 1716, in <lambda>
  File "/ffp/lib/python2.7/distutils/core.py", line 152, in setup
    dist.run_commands()
  File "/ffp/lib/python2.7/distutils/dist.py", line 953, in run_commands
    self.run_command(cmd)
  File "/ffp/lib/python2.7/distutils/dist.py", line 972, in run_command
    cmd_obj.run()
  File "build/bdist.linux-armv5tejl/egg/setuptools/command/easy_install.py", line 211, in run
  File "build/bdist.linux-armv5tejl/egg/setuptools/command/easy_install.py", line 422, in easy_install
  File "build/bdist.linux-armv5tejl/egg/setuptools/command/easy_install.py", line 478, in install_item
  File "build/bdist.linux-armv5tejl/egg/setuptools/command/easy_install.py", line 519, in process_distribution
  File "build/bdist.linux-armv5tejl/egg/pkg_resources.py", line 563, in resolve
  File "build/bdist.linux-armv5tejl/egg/pkg_resources.py", line 799, in best_match
  File "build/bdist.linux-armv5tejl/egg/pkg_resources.py", line 811, in obtain
  File "build/bdist.linux-armv5tejl/egg/setuptools/command/easy_install.py", line 446, in easy_install
  File "build/bdist.linux-armv5tejl/egg/setuptools/command/easy_install.py", line 476, in install_item
  File "build/bdist.linux-armv5tejl/egg/setuptools/command/easy_install.py", line 655, in install_eggs
  File "build/bdist.linux-armv5tejl/egg/setuptools/command/easy_install.py", line 930, in build_and_install
  File "build/bdist.linux-armv5tejl/egg/setuptools/command/easy_install.py", line 919, in run_setup
  File "build/bdist.linux-armv5tejl/egg/setuptools/sandbox.py", line 62, in run_setup
  File "build/bdist.linux-armv5tejl/egg/setuptools/sandbox.py", line 105, in run
  File "build/bdist.linux-armv5tejl/egg/setuptools/sandbox.py", line 64, in <lambda>
  File "setup.py", line 3, in <module>
    u"Tahoe-LAFS does not run under Python 3. Please use a version of Python between 2.4.4 and 2.7.x inclusive."
  File "/tmp/easy_install-TCxz5d/Nevow-0.10.0/nevow/__init__.py", line 5, in <module>
  File "/tmp/easy_install-TCxz5d/Nevow-0.10.0/nevow/_version.py", line 2, in <module>
ImportError: No module named twisted.python

Also, Nevow is unmaintained, or barely maintained. But it turns out (thanks to Daira for pointing this out to me) that twisted.web.templates is the successor to nevow!

​http://twistedmatrix.com/documents/current/web/howto/twisted-templates.html

So, we can reduce our dependencies by one, by using twisted.web.templates instead of Nevow!