Upgrade to upstream v1.13.7

• Upgrade to v1.13.7 (see !1 (merged) and !2 (merged) for the changes to upstream and pristine-tar)
• Switch to using Python 3 (#938352)
• Refresh shebang patch

I also did some routine maintenance tasks while I was at it:

• Declare compliance with policy v4.4.1. (No change needed.)
• Upgrade to dh compatibility level 12, use Build-Depends: debhelper-compat rather than old-style debian/compat.