bedevere did not remove `needs backport` label

[Mariatta]

PR python/cpython#5500 is a backport for python/cpython#5265

Bedevere should have removed the needs backport PR from python/cpython#5265.

[Mariatta]

So it was because the PR title starts with a space:

"title": " [3.6] bpo-32614: Modify re examples to use a raw string to prevent wa…  …rning (GH-5265)"

The regex pattern in https://github.com/python/bedevere/blob/master/bedevere/backport.py#L11 did not handle that 😅

[Mariatta]

Another example: python/cpython#5037 is the backport for python/cpython#5034. Labels not removed from python/cpython#5034

[Mariatta]

Another example from today: python/cpython#5746 is the backport for python/cpython#5726

[Mariatta]

For python/cpython#5746, it was because the commit message on python/cpython#5726 was wrong: it did not have the correct bpo- and GH- numbers.

[Mariatta]

Another instance where backport pr was not linked correctly:
python/cpython#6807

[brettcannon]

It looks like

bedevere/bedevere/backport.py

Line 11 in b41fe6b

TITLE_RE = re.compile(r'\[(?P<branch>\d+\.\d+)\].+?(?P<pr>\d+)\)')

is too strict. I bet we can change it to:

TITLE_RE = re.compile(r'\s*\[(?P<branch>\d+\.\d+)\].+\(?:GH-|#)(?P<pr>\d+)\)')

to fix this. This will:

1. Not care about leading whitespace (\s*)
2. Always match on the last set of parentheses (.+)
3. Require it be (GH- or (# ((?:GH-|#))