tests: put some Python 3 polish on inline Python invocations
A couple of these became inline python sessions because they got more
involved, and one of them is super-annoying and writes directly to
sys.stdout.buffer because I just couldn't make it work any other way.
Differential Revision: https://phab.mercurial-scm.org/D4230
.. Common link and substitution definitions.
.. |hg(1)| replace:: **hg**\ (1)
.. _hg(1): hg.1.html
.. |hgrc(5)| replace:: **hgrc**\ (5)
.. _hgrc(5): hgrc.5.html
.. |hgignore(5)| replace:: **hgignore**\ (5)
.. _hgignore(5): hgignore.5.html