python3Packages.numba: clean up dependencies
python3Packages.numba: clean up dependencies

Since the package is declared to be incompatibile with python versions older than 3.6 the `lib.optionals isPy27 [ funcsigs singledispatch]` part is redundant.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs