typing: fix a syntax error in mercurial/cext/bdiff.pyi
I noticed because `merge-pyi` on the non-cext implementations with this file as
input skipped the return type for this. `pytype-single --parse-pyi` confirmed
it was a problem.
#require test-repo pylint
Run pylint for known rules we care about.
-----------------------------------------
There should be no recorded failures; fix the codebase before introducing a
new check.
Current checks:
- W0102: no mutable default argument
$ touch $TESTTMP/fakerc
$ pylint --rcfile=$TESTTMP/fakerc --disable=all \
> --enable=W0102,C0321 \
> --reports=no \
> --ignore=thirdparty \
> mercurial hgdemandimport hgext hgext3rd | sed 's/\r$//'
Using config file *fakerc (glob) (?)
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------------------------------------* (glob) (?)
Your code has been rated at 10.00/10* (glob) (?)
(?)