Convert Demography.AGE_RANGE_LOOKUP to read-only mapping

[matt-graham]

Fixes issue described in #763 (comment) which was causing intermittent failure of tests/test_demography.py::test_py_calc test under Pandas v2.0.0.

Wraps the class attribute AGE_RANGE_LOOKUP of the Demography module, previously set to a defaultdict returned by the create_age_range_lookup function in src/tlo/util.py, in a read-only MappingProxyType. The defaultdict is first converted to a dict as otherwise the MappingProxyType will still use the underlying defaultdict for item access, which means the problem of accessing keys which are not present in the defaultdict adding new entries would remain.