added repo caching to noxfile

noxfile.py

@@ -1,9 +1,42 @@
 import pathlib
 import tempfile
+import typing
 
 import nox
 
 
+class RepoCache:
+    """class for caching checkouts of downstream repos
+
+    By caching the checkouts for downstream test runs,
+    the nox invokation is faster and there is less load
+    on the git server and network.
+    """
+
+    _checkouts: typing.ClassVar = {}
+
+    @classmethod
+    def clone(cls, session: nox.Session, repo: str) -> str:
+        """clones a git repo only once
+
+        Arguments:
+            session: the nox session in use
+            repo: a git repo url
+
+        Returns:
+            path to a temporary directory containing the
+            cloned git repo
+        """
+        tmpdir = cls._checkouts.get(repo, None)
+
+        if tmpdir is None:
+            tmpdir = tempfile.TemporaryDirectory()
+            session.run("git", "clone", repo, tmpdir.name, external=True)
+            cls._checkouts[repo] = tmpdir
+
+        return tmpdir.name
+
+
 @nox.session(python=["3.9", "3.10", "3.11"])
 def tests(session):
     session.install(".[test]")
@@ -19,11 +52,9 @@ def tests(session):
     ],
 )
 def downstream(session, repo, test_dir):
-    session.install(".")
+    tmpdir = RepoCache.clone(session, repo)
-
-    with tempfile.TemporaryDirectory() as tmpdir:
-        session.run("git", "clone", repo, tmpdir, external=True)
 
+    session.install(".")
     session.install(f"{tmpdir}[test]")
 
     test_path = pathlib.Path(tmpdir) / test_dir