Create/update python3-pysvn package

swh-loader-svn depends on pysvn (pysvn.tigris.org). Debian stable has python-pysvn (now python-svn) package but this is python2 support. pysvn supports python3 but is not included in this package yet.

This is create or update depending on the current package's status.

url: https://anonscm.debian.org/cgit/python-modules/packages/pysvn.

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Migrated from T363 (view on Phabricator)