Use relative paths for compat tests

The absolute paths were breaking some Azure conda-forge runners because
abspath was prefixing the path with `$SRC_DIR` which did not get
expanded on linux.

python/tests/compat/conftest.py

@@ -146,7 +146,9 @@ def assert_read(self, sym : str, df) -> None:
 def old_venv(request):
     version = request.param
     path = os.path.join("venvs", version)
-    requirements_file = os.path.abspath(os.path.join("tests", "compat", f"requirements-{version}.txt"))
+    # The requirements_file needs to be relative to the [path] we use for the venv.
+    # Absolute paths break some Azure CI runners on conda forge
+    requirements_file = os.path.join("..", "..", "tests", "compat", f"requirements-{version}.txt")
     with Venv(path, requirements_file, version) as old_venv:
         yield old_venv