Revert "[#5973] crazy pysvn hacking so that packaging tools know it is in the venv"

This reverts commit dcf934e01089bc71a81939009abeb6145373e675.

Let's handle pysvn simply instead of being tricky.
Revert "[#5973] Make the pysvn dep explicit, so we fail sooner (setup.py develop instead of runtime import)"

This reverts commit b0c97b952b3671f0e45c7f030ba920bf280abffa.

Let's handle pysvn simply instead of being tricky.
[#5973] simple pysvn import check
[#5973] remove another reference to system-site-packages