Force an error on "sklearn" installation attempts

According to "sklearn"'s [description][0], installing the package
doesn't always raise an error, but only according to a specific
schedule. Obviously this doesn't work for tests. Fortunately setting the
`SKLEARN_ALLOW_DEPRECATED_SKLEARN_PACKAGE_INSTALL` environment variable
to `False` always forces the error.

[0]: https://pypi.org/project/sklearn/#description

noxfile.py

@@ -65,5 +65,6 @@ def build(session):
 def install(session):
     """Install the package."""
     setdefaults(session)
+    session.env["SKLEARN_ALLOW_DEPRECATED_SKLEARN_PACKAGE_INSTALL"] = "False"
     session.run("python", "-mpip", "install", "--upgrade", "pip")
     session.run("python", "-mpip", "install", *Path("dist").glob("*.whl"))