pythonPackages.pint: add missing dependencies
pythonPackages.pint: add missing dependencies

importlib-metadata and packaging appear to have been falsely satisfied
by the inclusion of pytestCheckHook in checkInputs. this can be
demonstrated by attempting to build this package with doCheck = false.

also removed the isPy27 clause as package no longer seems to support <3.6
anyway now.

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