Subvertpy 0.11 introduced some new logic in client.propget() that

normalizes a path to an absolute path. This in turn triggers an eventual

assertion failure in libsvn, which checks to make sure a full repository

URL wasn't passed in (somewhere). This was not a problem in Subvertpy

0.10 (which also didn't have this path normalization).
The problem applies only to propget(), and thankfully does not apply

to proplist(). To work around this crash, we're now calling

proplist() and filtering results from that. This method avoids the

path normalization that broke us, and provides the same results.
The Subvertpy bug is tracked at: https://github.com/jelmer/subvertpy/issues/35

Unit tests pass on Python 2 and 3, against Subverpty 0.10 and 0.11.