Fix mypy type errors and add mypy to .drone.yml

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.

1 files changed, 9 insertions, 0 deletions

diff --git a/tox.ini b/tox.ini
index 2fd97716..0615a0b3 100644
--- a/tox.ini
+++ b/tox.ini
@@ -24,3 +24,12 @@ commands =
 deps =
 	-rrequirements.txt
 	-rrequirements.dev.txt
+
+[testenv:mypy]
+basepython =
+    python3.7
+commands =
+    {envpython} -m mypy rdflib --show-error-context --show-error-codes
+deps =
+	-rrequirements.txt
+	-rrequirements.dev.txt