Add coverage.py multiprocessing config

https://coverage.readthedocs.io/en/coverage-5.3/config.html

> concurrency (multi-string, default “thread”): the name concurrency
> libraries in use by the product code. If your program uses
> multiprocessing, gevent, greenlet, or eventlet, you must name that
> library in this option, or coverage.py will produce very wrong results.

pyproject.toml

@@ -1,3 +1,6 @@
+[tool.coverage.run]
+concurrency = ["multiprocessing"]
+
 [tool.isort]
 profile = "black"
 src_paths = ["inboard", "tests"]