It was removed (probably by mistake) in
51e78fc5fa21870d415c52f90b93e3c6ad57be46.
gdb/ChangeLog:
2021-06-03 Hannes Domani <ssbssa@yahoo.de>
* python/py-symbol.c (gdbpy_initialize_symbols): Restore
gdb.SYMBOL_LABEL_DOMAIN constant.
gdb/testsuite/ChangeLog:
2021-06-03 Hannes Domani <ssbssa@yahoo.de>
* gdb.python/py-symbol.exp: Test symbol constants.