Make 0.x versions of jupyterlite-xeus-python compatible with JupyterLite 0.2.0

[jtpio]

The update to JupyterLite 0.2.0 was done in #155.

There isn't many breaking changes in JupyterLite 0.2.0 that affect jupyterlite-xeus-python.

Except for the removal of ENV_EXTENSIONS from the top-level jupyterlite_core.addons.federated_extensions:

xeus-python-kernel/jupyterlite_xeus_python/env_build_addon.py

Lines 25 to 28 in b00d80d

	from jupyterlite_core.addons.federated_extensions import (
	FederatedExtensionAddon,
	ENV_EXTENSIONS,
	)

Maybe it could be useful to enable jupyterlite-xeus-python==0.x to work with JupyterLite 0.2.0, in case #164 or emscripten-forge/recipes#643 would prevent some users from upgrading.

[jtpio]

This could be done here in the 0.x branch by adding a try / except block.