This fixes a regression which was introduced with support for toc_depth.
Relevant tests have been moved and updated to the new framework.
Fixes #1107.

The test framework also received an addition. The assertMarkdownRenders
method now accepts a new keyword expected_attrs which consists of a dict
of attrs and expected values. Each is checked against the attr of the
Markdown instance. This was needed to check the value of md.toc and
md.toc_tokens in some of the included tests.

* Pygments specific tests now only run when the pygments version installed
  matches the expected version. That version is defined in an environment
  variable (PYGMENTS_VERSION) in the 'pygments' tox env (see #1030).
* When the Python lib tidylib is installed but the underlying c lib is not,
  the relevant tests are now skipped rather than fail. This matches the
  behavior when the Python lib is not installed. The tox envs are now useful
  on systems which don't have the c lib installed.

If the Python stack comes within 100 frames of the recursion limit,
then the nesting limit of blockquotes is met. Any remaining text,
including angle brackets, are simply wrapped in a paragraph. To
increasing the nesting depth, increase Python's recursion limit.
However, be aware that each level of recursion will likely result in
multiple frames being added to the Python stack. Therefore, the
recursion depth and nesting depth are not one-to-one.

Performance is an concern here. However, the current solution seems like
a reasonable compromise. It doesn't slow things down too much, but also
avoids Markdown input resulting in an error. This is mostly only a concern
with contrived input anyway. For the average Markdown document, this will
likely never be an issue.

Fixes #799.

Closes #913

* Python syntax upgraded using `pyupgrade --py3-plus`
* Travis no longer uses `sudo`. See https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration

See #760 for Python Version Support Timeline and related dicussion.

Fixes #435.

All file-based tests are now defined as unittest test cases via a
metaclass which walks a directory and builds a unittest for each pair
of test files.

To run the tests just run `python -m unittest discover tests`. Or use
tox as the tox config has been updated to run the new tests and all nose
specific code has been removed.

The test generator tools have been removed as well. If any changes or
additions need to be made to tests, they should be implemented using
the new framework rather than with the file-based tests. Eventually,
only the PHP and pl tests should remain as file-based tests.