Display warning when user has third-party dataclasses/typing installed

Hopefully makes the solution to #10 more obvious.

gobbli/__init__.py

@@ -1,10 +1,26 @@
+import warnings
+
+import pkg_resources
+
 import gobbli.augment as augment
 import gobbli.dataset as dataset
 import gobbli.experiment as experiment
 import gobbli.io as io
 import gobbli.model as model
 from gobbli.util import TokenizeMethod
 
+# Warn the user of potential conflicts using the old third-party typing/dataclasses
+# modules
+for conflicting_pkg in ("typing", "dataclasses"):
+    req = pkg_resources.Requirement.parse(conflicting_pkg)
+    if pkg_resources.working_set.find(req) is not None:
+        warnings.warn(
+            f"You've installed a third-party module named '{conflicting_pkg}' which "
+            "conflicts with a standard library module of the same name.  This can cause "
+            "errors when unpickling code, e.g. when running experiments using Ray. Consider "
+            f"uninstalling the module:\n\npip uninstall {conflicting_pkg}"
+        )
+
 __all__ = [
     # Modules
     "augment",