This reverts commit ca8695b9.

* Specify the inclusion of test benchmark images and examples using
  `include` commands in the MANIFEST file.
* Use setup.cfg file to ensure COPYING file is included in dist tarball
  and pytest is specified as the test runner.

* Tests work by comparing produced image against the respective
  benchmark image included in sources.
* Specify test dependencies in setup.py: pytest, cv2, and skimage.
* Update url in setup.py.
* Cosmetic cleanups in setup.py.

Allow exporting to PNG and JPG using the pyx.canvas.writeGSfile()
interface. Additional parameters 'resolution' and 'device' to draw()
allows finetuning the exported bitmap file. Print warnings if resolution
and/or device parameters are passed when exporting as eps or pdf where
they have no meaning (vector formats).

Python3 drops support for __cmp__() based comparison and instead
requires the specification of rich comparison methods for objects.
This commit implements the six comparison methods (eq, ne, lt, le, gt,
and ge) for objects of the Visible class and drops the __cmp()__ method
entirely.

Pass arg `enlargement` in pt units as the optional arg
"page_bboxenlarge" to pyx.writetofile instead of specifying bbox
directly. This fixes thick strokes being clipped at the image edge which
happens when the bbox is specified directly and enlargement is set to 0
by default.

Set the default value of `enlargement` to 1 (instead of 0) to make it
consistent with pyx.writetofile defaults.

* Prevent lines from running off into blobs.
* Join intersecting line and cycloid at beginning and end points.

* Define helper function _selfintersections that returns the points at
  which the curve intersects itself. Implementation taken from
  pyx.deformer.parallel.normpath_selfintersections in PyX 0.12.
* Replace calls to normpath_selfintersections by _selfintersections for
  3D decorated lines (Gluon and Gluino); remove parallel transformation
  that is no longer needed to determine _selfintersections.

pyx.normpath does not support curveradius methods as of version 0.13.

This works around a hepnicenames issue where it does not print some
identifiers in upshape with TeXLive 2019 and newer, and italicised
particle identifiers look better in feynman diagrams in any case.

Use
* TexEngine and LatexEngine instead of TexRunner and LatexRunner
  respectively.
* text.defaulttextengine instead of text.defaulttexrunner.
* text.defaulttextengine instead of text.default_runner.