Squashed 'external/devlib/' changes from a9fcc75f6..7866086ec

7866086ec shutils: Remove shebang
734b24987 target: Run shutils.in in busybox
23f663d14 target: Fix AndroidTarget pickling
235422855 devlib: Use async Target API
be593120a target: Expose Target(max_async=50) parameter
bec2c02fd target: Enable async methods
fba315e76 utils/async: Add new utils.async module
981b69ff9 setup.py: Require Python >= 3.7
49e452046 setup.py: cleanup dependencies in setup.py
d0b5c1a2a target: Make __getstate__ more future-proof
dcfc28e3f target: Fix Target.get_connection()'s busybox
16c3f0e98 utils.misc: Make nullcontext work with asyncio

git-subtree-dir: external/devlib
git-subtree-split: 7866086eccdc9781df2a806a05e7424723cb9d8d