CDRIVER-4795 restore `MONGOC_MD_FLAG_ENABLE_ICU` (#1494)

* support Python 3 in `parse_handshake_cfg.py`

* add numeric assignments to enum values

* format `mongoc-handshake-private.h`

* fix reference to test function

* restore `MONGOC_MD_FLAG_ENABLE_ICU`

* comment about `MONGOC_MD_FLAG_ENABLE_ICU` removal