[MNT] [Dependabot](deps-dev): Update polars requirement from <0.20.0 …

…to <0.21.0 (#176)

Updates the requirements on [polars](https://github.com/pola-rs/polars)
to permit the latest version.

pyproject.toml

@@ -56,7 +56,7 @@ all_extras = [
     "mapie",
     "matplotlib>=3.3.2",
     "ngboost",
-    "polars<0.20.0",
+    "polars<0.21.0",
     "pyarrow<14.0.0; python_version < '3.12'",
     "statsmodels>=0.12.1",
     "tabulate",