Add docs and test extras

This makes it easier to install just documentation or test dependencies
without installing all dev dependencies.  It also enforces version
restrictions during nox runs.

noxfile.py

@@ -18,12 +18,11 @@ def check_example(session: nox.Session):
 
 @nox.session(reuse_venv=True)
 def docs(session: nox.Session):
-    session.install(".", "Sphinx")
+    session.install(".[docs]")
     session.run("sphinx-build", "docs", "docs/_build")
 
 
 @nox.session(python=["3.6", "3.7", "3.8", "3.9", "3.10", "pypy3"], reuse_venv=True)
 def test(session: nox.Session):
-    session.install(".")
-    session.install("pytest", "hypothesis", "pytz")
+    session.install(".[test]")
     session.run("pytest", "-vv", "--tb=short", "--log-level=INFO")

pyproject.toml

@@ -11,6 +11,10 @@ documentation = "https://pyiso8601.readthedocs.io/en/latest/"
 
 [tool.poetry.dependencies]
 python = ">=3.6.2,<4.0"
+pytest = { version = "^6.2.2", optional = true }
+hypothesis = { version = "^6.1.1", optional = true }
+pytz = { version = "^2021.1", optional = true }
+Sphinx = { version = "^4.2.0", optional = true }
 
 [tool.poetry.dev-dependencies]
 mypy = "^0.910"
@@ -24,6 +28,10 @@ pre-commit = "^2.15.0"
 nox = "^2021.10.1"
 Sphinx = "^4.2.0"
 
+[tool.poetry.extras]
+docs = ["Sphinx"]
+test = ["pytest", "hypothesis", "pytz"]
+
 [build-system]
 requires = ["poetry-core>=1.0.0"]
 build-backend = "poetry.core.masonry.api"