tests/test-check-pylint.t

py3: use stringutil.forcebytestr() instead of str()

We need to convert errors to bytes using stringutil.forcebytestr()

Differential Revision: https://phab.mercurial-scm.org/D3324

#require test-repo pylint hg10

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
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