doc generation failing due to typing imports

[fmigneault]

When building docs with an sphinx-argparse directive, the following error occurs.

[...]
  File "/home/francis/dev/miniconda/envs/weaver-py3/lib/python3.7/site-packages/sphinxarg/ext.py", line 474, in run
    exec(code, mod)
  File "../weaver/cli.py", line 28, in <module>
    from weaver.typedefs import CWL, HeadersType, JSON
  File "/home/francis/dev/weaver/weaver/typedefs.py", line 34, in <module>
    from weaver.processes.wps_process_base import WpsProcessInterface
  File "/home/francis/dev/weaver/weaver/processes/wps_process_base.py", line 17, in <module>
    from weaver.typedefs import CWL_RuntimeInputsMap
ImportError: cannot import name 'CWL_RuntimeInputsMap' from 'weaver.typedefs' (/home/francis/dev/weaver/weaver/typedefs.py)

where my directive is as follows:

.. argparse::
    :module: weaver.cli
    :func: make_parser
    :prog: weaver

The encountered error happens due to a typing import.
In the weaver.cli module, there is this in the imports:

from typing import TYPE_CHECKING
if TYPE_CHECKING:
    from weaver.typedefs import CWL, HeadersType, JSON

Because this is within a typing-enabled block, it should be ignored for "normal" execution.
Only typing parsers/linters must consider those imports, which I believe arparse shouldn't consider.

Note, this might be caused by interactions with package sphinx_autodoc_typehints which uses those typings during documentation generation. I'm not familiar enough with the internals of sphinx building to know if sphinx-argparse could disable typings only for its own context build.