WIP: community/py3-pip: make it easier to use as a pip517 builder

• community/py3-pip: make it easier to use as a pip517 builder
• community/py3-pip: disable testsuite to reduce dependency load
• community/py3-pip: add fish completion
• community/py3-pip: update comments on certifi
• community/py3-pip: use default shell completion functions
• main/fish: drop pip completion
• community/py3-flit_core: new aport
• community/flit: use py3-flit_core, replace pytoml with toml