get_gravatar_url_for_email() has to build a query string to pass to

the Gravatar services, based on provided arguments and project settings.

This is accomplished by constructing a QueryDict, passing in any and

all query string arguments we need, and then encoding it onto the end of

a URL.
The problem with this is that QueryDict encodes in dictionary

iteration order, which is not consistent across Python versions. There's

no way to sort to this. This makes it very hard to test payloads

containing a Gravatar URL in any sane way.
This change ditches QueryDict and just assembles our own ordered list

of query string key/value pairs, which we then pass to Python's own

urlencode(). This ensures that we always have a consistent order to

test with.

Djblets unit tests pass on all supported versions of Python.
Verified this fixed a unit test on Review Board when running Python 3.5.