Release Djblets 0.7.23.
Bump to 0.7.24 dev.
Merge branch 'release-0.7.x'
Fix breakages with the webapi unit tests.

These unit tests have been broken a little while, due to the old
mimetypes lists being removed. They've been fixed to test the new
variables.
Abstract away some types

String and integer types have changed in python 3. In 2.x, the 'str' type is a
string of bytes, and the 'unicode' type is a string of text. In 3.x, the
'bytes' type has replaced the old 'str', and 'str' has replaced 'unicode'. The
six library has abstractions for these that we can use while we're trying to
support both.

Additionally, 3.x has merged the 'int' and 'long' types, so there's a similar
abstraction for that.

Testing done: Ran unit tests.
Reviewed at https://reviews.reviewboard.org/r/4906/