Allow passing through arguments to pytest

Can enable running individual tests or supllying any other arguments.
Will remove the default argument `--cov`.

noxfile.py

@@ -3,6 +3,7 @@ import nox
 
 @nox.session(python=["3.7", "3.8", "3.9"])
 def tests(session):
+    args = session.posargs or ["--cov"]
     session.run("poetry", "install", external=True)
-    session.run("pytest", "--cov")
+    session.run("pytest", *args)
     pass

pyproject.toml

@@ -15,7 +15,6 @@ click = "^8.0"
 [tool.poetry.dev-dependencies]
 pytest = "^6.2"
 coverage = {extras = ["toml"], version = "^6.2"}
-
 pytest-cov = "^3.0.0"
 pytest-mock = "^3.6.1"