0.13.0

Changes

• pytest-xdist pytest extension is now installed so tests can be
  run in parallel.
• CI now builds manylinux2010 and manylinux2014 binary wheels
  instead of a mix of manylinux2010 and manylinux1.
• Official support for Python 3.8 has been added.
• Bundled zstandard library upgraded from 1.4.3 to 1.4.4.
• Python code has been reformatted with black.