Add testing on macOS, update GitHub Actions, add Dependabot

[agriyakhetarpal]

Seeing if we can run the test suite on macOS, in light of #55. Also adds Dependabot and other security-related things.

[agriyakhetarpal]

The tests on macOS failed as follows:

FAILED test/test_repair.py::test_repair_rpath[wheel_file0-libname0-expected0] - AssertionError: expected runtime path $ORIGIN not found in libgeos_c.so
assert '$ORIGIN' in ['$ORIGIN/../../../../../../../../../var/folders/8d/778wjbv96mq1760tv6gk374m0000gn/T/tmp0kceidi2/Shapely-1.8.2/Shapely.libs']
 +  where ['$ORIGIN/../../../../../../../../../var/folders/8d/778wjbv96mq1760tv6gk374m0000gn/T/tmp0kceidi2/Shapely-1.8.2/Shapely.libs'] = Dylink(mem_size=2384, mem_align=4, table_size=4, table_align=0, needed=['libgeos.so.3.10.3'], export_info={}, import_info={}, runtime_paths=['$ORIGIN/../../../../../../../../../var/folders/8d/778wjbv96mq1760tv6gk374m0000gn/T/tmp0kceidi2/Shapely-1.8.2/Shapely.libs']).runtime_paths

And they are now passing when I added my fix from #55 here. Let's merge that one first; I'll rebase this later!