Errors were being piped to true, but that will fail if it runs with
pipefail. The better option is to do `black ... || true` which will work
for ignoring errors even with pipefail.

This patch replace all uses of nose with pytest. It also includes a
pytest plugin for creating EARL reports for tests with a `rdf_test_uri`
parameter.

Some caveats:

- HTML report directory is now htmlcov instead of coverage
- There is some warning related to the EARL reporting plugin which I can't quite figure out:
  ```
  .venv/lib64/python3.7/site-packages/_pytest/config/__init__.py:676
    /home/iwana/sw/d/github.com/iafork/rdflib/.venv/lib64/python3.7/site-packages/_pytest/config/__init__.py:676: PytestAssertRewriteWarning: Module already imported so cannot be rewritten: test.earl
      self.import_plugin(import_spec)
  ```
  This is not causing any problems as far as I can tell, but still annoying.
- python setup.py test won't work anymore, I can make it work but this
  is not advised by pytest:
  https://github.com/pytest-dev/pytest-runner/#deprecation-notice
- run_test.py is still there but it's not really referenced anymore from
  anywhere and the options it accepts are completely different as it's options
  were based on nose. I would say it should be removed entirely but for now
  it is basically just a wrapper around pytest that basically does nothing.
- Removed references to test attributes as currently they are not being
  used anywhere anyway, I guess we can add them back if there is some
  use for them later.
- A lot of tests are still marked to skip when really they should be marked
  with xfail. This is also affecting the RDFT test manifests and result in
  reports saying tests are skipped when really we know they will fail and
  they are only skipped for this reason. But there is no change here from
  before, and pytest makes it easier to dynamically do expected failures.

Special thanks to Wes Turner for his advice and inputs on this process.

Fixes #1311

Add `mypy` to .drone.yml and fix type errors that come up.

Not type checking examples or tests.

Other changes:

- fix return value for `Graph.serialize` (refs #1394)
- remove default value for
  `rdflib.plugins.sparql.algebra.translateAlgebra` (refs #1322)
- add .dockerignore to reduce context size and make docker quicker
  to run.
- add .flake8 config to ignore line length as black is managing
  formatting.
- add mypy to docker-compose, makefile and tox.ini
- fix the way csv2rdf is invoked to ensure that the right code gets
  executed.

remove coveralls install on python3.8 tests

Replaces #1246
Unfortunately the code duplication is required in this case. Drone doesn't have quite as sophisticated matrix runner functionality that Travis has. See here: https://docs.drone.io/pipeline/docker/examples/languages/python/
We could parametrically create the runners using a jsonnet file (as shown at the bottom of that example page) but I wanted to keep it all in the .yml file if possible.