The SVN client was handling a variety of things using the wrong string

types, most notably filenames. In Python 3, filenames are expected to be

treated as str internally, and then serialized/deserialized using the

filesystem encoding (theoretically this is also the case in Python 2,

but the implicit casting makes it easy to be sloppy). This change fixes

those up so that we're at least sucessfully running the tests.
This also incidentally fixes bug 4547, which involves using

Popen.communicate() instead of Popen.wait().

Ran unit tests on Python 2.7 and 3.6.