Use pytest-xdist to speed up tests (#1290)

* Run tests in CI using pytest-xdist
* Documentation for pytest-xdist

Closes #1289

setup.py

@@ -70,6 +70,7 @@ setup(
         "docs": ["sphinx_rtd_theme", "sphinx-autobuild"],
         "test": [
             "pytest>=5.2.2,<6.3.0",
+            "pytest-xdist>=2.2.1,<2.3",
             "pytest-asyncio>=0.10,<0.15",
             "beautifulsoup4>=4.8.1,<4.10.0",
             "black==20.8b1",