Move on from deprecated pkg_resources api

[farhan]

Shift the code from pkg_resources to importlib.resources api

Migration guide: https://importlib-resources.readthedocs.io/en/latest/migration.html
Decision making story: openedx/XBlock#676
Reference PR to take learning from: https://github.com/openedx/xblock-sdk/pull/350/files

Prerequisites:

• Support of python 3.8 should be dropped. Relevant epic

Acceptance Criteria:

• Make sure the repository is using xblock version 5.0.0 or above. Upgrade the version if required, preferably via the make upgrade command.
• There should be no usage of pkg_resources in the repository
• Test cases should be passed
• XBlock should perform all of its functionality related to change (loading resources)

[bmtcril]

This is another one that's blocked until we do our Aspects v2 breaking changes, until then we still need to support 3.8 for named releases back to Nutmeg.