add pythonImportsCheck and testers.testVersion
started failing on staging-next with `No module named pip` and after
adding pip
aws-google-auth> ir_d = dist.fetch_build_eggs(dist.install_requires)
aws-google-auth> WARNING: Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.HTTPSConnection object at 0x7ffff5542c10>: Failed to establish a new connection: [Errno -3] Temporary failure in name resolution')': /simple/urllib3/
aws-google-auth> ERROR: Could not find a version that satisfies the requirement urllib3<1.27,>=1.25.4 (from versions: none)
checkInputs used to be added to nativeBuildInputs. Now we have
nativeCheckInputs to do that instead. Doing this treewide change allows
to keep hashes identical to before the introduction of
nativeCheckInputs.
