tests/test-gendoc-pt_BR.t

py3: use .startswith() instead of bytes[0]

Doing bytes[0] will return the ascii value of that position which breaks
comparison with a bytechar.

This makes test-absorb.t work again on py3.

Differential Revision: https://phab.mercurial-scm.org/D6508

#require docutils gettext

  $ $TESTDIR/check-gendoc pt_BR
  checking for parse errors