cast is used in internal.io_handling but not imported if typing is not present

[mattsb42-aws]

As with all of our other modules, in internal.io_handling we catch all imports from typing because of issues with CPython 3.5.0 and 3.5.1.

aws-encryption-sdk-cli/src/aws_encryption_sdk_cli/internal/io_handling.py

Lines 30 to 35 in ba1f7ea

	try: # Python 3.5.0 and 3.5.1 have incompatible typing modules
	from typing import cast, Dict, IO, List, Type, Union # noqa pylint: disable=unused-import
	from aws_encryption_sdk_cli.internal.mypy_types import SOURCE, STREAM_KWARGS # noqa pylint: disable=unused-import
	except ImportError: # pragma: no cover
	# We only actually need these imports when running the mypy checks
	pass

In 1.1.6, we removed the dependency on typingbecause I had thought that we only used imports from typing in our Python-2-compatible typehints.

However, in internal.io_handling, we use typing.cast.

aws-encryption-sdk-cli/src/aws_encryption_sdk_cli/internal/io_handling.py

Line 270 in ba1f7ea

stream_args=stream_args, source=cast(IO, source), destination_writer=destination_writer

This only does anything when the file is being processed by mypy or a similar analyzer, so a quick fix for this is for us to just assign a no-op lambda to cast in the import catch statement.

However, I am concerned that our CI did not catch this...I would like to find out what happened there too.

[mattsb42-aws]

I am unable to replicate the error that was reported, but in light of this and how we are actually using the typing module, I think that the correct solution here is to simply remove the install dependency. This will require us to add a no-op definition for cast in the modules that we use it, as well as for any of the types that we include in the calls to cast.

ex:

except ImportError:
    cast = lambda typ, val: val
    IO = Tuple = None