Cap the pyparsing dependency at 2.4.x.
Cap the pyparsing dependency at 2.4.x.

`pyparsing` is needed by a couple different dependencies, including
setuptools, but versions newer than 2.4.x only support Python 3.

This change caps us at 2.4.x, in order to avoid any breakages during
installation.

We should now have every potentially-unsafe dependency included in a
default install of Review Board capped to a safe range. That should help
keep Review Board 3.0.x installable long-term, provided compatible
versions of Python and `pip`/`easy_install`.

Testing Done:
Installed the package in a fresh virtualenv. Verified that it installed
without issues and that functionality seemed to work without errors.

Went through `site-packages` and checked each package, ensuring that any
that are a direct or indirect dependency of Review Board was capped,
explicitly a 2.7 backport of a Python 3 feature, or advertises long-term
support for Python 2.7.

Reviewed at https://reviews.reviewboard.org/r/11595/

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