bup: use system Python, update vendored dependencies

[iMichka]

• Have you followed the guidelines for contributing?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
• Does your build pass brew audit --strict <formula> (after doing brew install <formula>)?

---

[dawidd6]

==> /usr/local/Cellar/bup/0.30_1/bin/bup init
/usr/local/Cellar/bup/0.30_1/bin/bup: /usr/local/Cellar/bup/0.30_1/libexec/bup.py: python3: bad interpreter: No such file or directory
/usr/local/Cellar/bup/0.30_1/bin/bup: line 2: /usr/local/Cellar/bup/0.30_1/libexec/bup.py: Undefined error: 0
Error: bup: failed

[dawidd6]

Changing to:

    ENV["PYTHON"] = Formula["python3"].opt_bin/"python3"

produces this during bup init:

error: bup may crash with python versions other than 2, or eat your data

overriding this error with BUP_ALLOW_UNEXPECTED_PYTHON_VERSION spits out:

Traceback (most recent call last):
  File "/home/linuxbrew/.linuxbrew/Cellar/bup/0.30_1/lib/bup/cmd/bup-init", line 6, in <module>
    from bup import git, options, client
  File "/home/linuxbrew/.linuxbrew/Cellar/bup/0.30_1/lib/bup/bup/git.py", line 685
    raise GitError, e, sys.exc_info()[2]
                  ^
SyntaxError: invalid syntax

which concludes that bup is not adapted to python3.

[Bo98]

There is some Python 3 work in place already for 0.31. I don't know if it'll be ready before the end of the year though.

[iMichka]

Let's go with system python then.