Remove long-deprecated assertEquals uses

These have been removed in Python 3.12:
https://docs.python.org/3/whatsnew/3.12.html#id3

Bug: 388344853
Fixes: 402292455
Test: atest fhir-spec-extractor-test -- --abi x86_64
Flag: TEST_ONLY
Change-Id: Ib0b4445e491ca4083114a2c8b60d0b42e7726870
Merged-In: Ib0b4445e491ca4083114a2c8b60d0b42e7726870