Add doctests to the test suite

- use xdoctest to validate code snippets in docstrings
- make xdoctest part of the nox session "test" via
  the new `test_docstrings()` test case
- add nox session "test-docstrings" for convenience;
  also, `xdoctest.doctest_module()` does not discover
  docstrings that are imported at the root of the package
  => each new module with docstrings must be added to
     `test_docstrings()` by hand, which is likely forgotten
  => the nox session "test-docstrings" should run on CI

noxfile.py

@@ -76,6 +76,7 @@ nox.options.reuse_venv = "no"
 nox.options.sessions = (  # run by default when invoking `nox` on the CLI
     "format",
     "lint",
+    "test-docstrings",
     f"test-{MAIN_PYTHON}",
 )
 nox.options.stop_on_first_error = True
@@ -156,6 +157,7 @@ def test(session: nox.Session) -> None:
         "pytest",
         "pytest-cov",
         "semver",
+        "xdoctest",
     )
 
     args = session.posargs or (
@@ -167,6 +169,16 @@ def test(session: nox.Session) -> None:
     session.run("pytest", *args)
 
 
+@nox_session(name="test-docstrings", python=MAIN_PYTHON)
+def test_docstrings(session: nox.Session) -> None:
+    """Test docstrings with `xdoctest`."""
+    start(session)
+    install_pinned(session, "xdoctest[colors]")
+
+    session.run("xdoctest", "--version")
+    session.run("xdoctest", "src/lalib")
+
+
 def start(session: nox.Session) -> None:
     """Show generic info about a session."""
     if session.posargs: