Just hope things are working as expected on my machine.

Signed-off-by: Stephen Finucane <stephen@that.guru>

There are three issues to resolve here, one with an input file and two
with reference PDFs.

rst2pdf/tests/input/test_issue_175.txt
  This attempts to insert a large enough spacer to force a header to the
  bottom of the first page and the text onto the next page, in order to
  ensure the header will be moved to the next page to group it with its
  text. However, the spacer wasn't large enough and led to both the
  header and text being included on the first page.

rst2pdf/tests/reference/test_aafigure.pdf
  It doesn't appear possible to format a backtick as a literal without
  including the escape character. The escape character wasn't included
  in the reference PDF.

rst2pdf/tests/reference/test_issue_288.pdf
  This contained a different number of block elements on Python 3 than
  on Python 2. Update the PDF but disable the build until Python 2 is
  dropped.

Signed-off-by: Stephen Finucane <stephen@that.guru>

Previously, we were assessing whether a PDF was correct by comparing a
checksum of the file against a list of known valid checksums. When a
different checksum was seen, which was quite often given the amount of
moving pieces here, it was up to the user to determine whether there was
a bug or simply a small but valid change in the output PDF. Let's
simplify this. Instead of storing checksums, use the pymupdf library to
compare the PDFs programmatically. We need to use a bit of fuzzing here,
since blocks can get moved around and text rewrapped, but it should be
mostly bullet proof otherwise.

Signed-off-by: Stephen Finucane <stephen@that.guru>

Resolve issues with the Sphinx tests.

Signed-off-by: Stephen Finucane <stephen@that.guru>

It seems there are issues with the Python 2.7 wheels for PyMuPDF [1]. We
could wait for a fix, but given the next version of rst2pdf is expected
to be Python 3 only it's as easy to just drop testing for this now.

[1] pymupdf/PyMuPDF#511

Signed-off-by: Stephen Finucane <stephen@that.guru>