remove warus operator from builder.py

[pracio]

the warus operator was introduced in Python 3.8, keeping it will break the support of Python 3.6.

py-lua-parser/luaparser/builder.py

Line 684 in cd08291

elif m := LUA_DOUBLE_SQUARE_RE.match(lua_str):

py-lua-parser/setup.py

Line 24 in cd08291

"Programming Language :: Python :: 3.6",

we should either drop support of Python 3.6 or remove the use of warus operator.