While Subvertpy is easier to build, it's less complete than PySVN, and

has been the source of several bugs. Some users who have tried switching

to PySVN from Subvertpy have had issues due to a version of Subvertpy

remaining on the system.
We now default to PySVN as the backend if it's installed. It's

better-tested and likely to be more compatible.
For Subvertpy support to work, a minimum version of 0.9.1 must be

installed, since older versions have caused errors we don't want to deal

with.
I also renamed 'imported_dependency' to 'has_svn_backend', which is a

bit more clear and less likely to result in conflicts.

Tested that PySVN was being used by default here.
Tested the version comparison with my installed version, and with various other

versions by hand.
Unit tests pass.