#### Why I did it
Fix a recent build error introduced by a pre-release redis-py. This is a general issue because `python setup.py install` (ie `easy_instal`) does not ignore pre-release versions. The fix is suggested by https://github.com/pypa/setuptools/issues/855#issuecomment-583803959
Redis 4.0.0b1 has been uploaded to pip as a prerelease version. This
version drops support for Python 2 and only supports Python 3. Because
setup.py is being run, it will use the latest version of a package and
not the latest stable version (which is still 3.5.3).
Therefore, pin the redis package to version 3.5.3, so that it will work
for both Python 2 and 3.
#### How to verify it
Make sure that redis-dump-load for Python 2 builds today.
