Log Message:
sysutils/bup: Update to 0.31

Passes bup's test suite on netbsd-8/amd64, with TZ=UTC (needed to work
around what is likely a bug in git around DST changes).

Packaging changes/notes:
  - bup depends directly on readline, GNU version required
  - man pages have not yet been updated

Upstream changes include many minor improvements and bugfixes,
and:

Notable changes in 0.31 (since 0.30.1)
======================================

* Python 3 is now supported, and Python 2 support is deprecated.  It's
  possible that we'll stop new development for Python 2 fairly soon.
  If so, we'll probably continue to fix bugs in the last Python 2
  compatible version for a while, but please make plans to migrate.

* `bup features` has been added.  It reports information about bup
  itself, including the Python version, and the current availability
  of features like readline or support for POSIX ACLs.

Build and install
-----------------

* `bup` itself is now located in now located in the cmd/ directory in
  the install tree and finds sub-commands, etc. relative to its own
  location.