Remove bcrypt and pyparsing pinned dependencies on Python 3.
Remove bcrypt and pyparsing pinned dependencies on Python 3.

We have a handful of dependencies that are actually dependencies of
other dependencies. To avoid installation issues on Python 2.7 systems,
where some of *our* dependencies ended up installing Python 3-only
packages, we began to pin some dependencies-of-dependencies in our
`setup.py`. When doing so, we also specified some minimum versions for
Python 3.

This actually made the Python 3 situation more complex, since we were
now fighting some of the version specifiers in some dependencies.
Namely, `bcrypt` and `pyparsing`. Really, we only wanted to pin on
Python 2.7 anyway.

These pinned dependencies have been updated to only pin on Python 2.7.
For Python 3, we now stay out of the picture, allowing our dependencies
to specify the versions of their dependencies that they want.

Testing Done:
Built packages and verified that the version specifiers for `bcrypt`
and `pyparsing` were only set for Python 2.7.

Verified that this avoided some installation issues.

Reviewed at https://reviews.reviewboard.org/r/11831/

reviewboard/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard