bug: robot server fails to start on robot with httpapiv2 because it relies on typing_extensions

[sfoster1]

Overview

#5208 imports typing_extensions at runtime, but typing_extensions is a dev dependency only for static type analysis and not present on robots. This means that the robot server crashes on start when http apiv2 is enabled.

Like in https://github.com/Opentrons/opentrons/blob/edge/api/src/opentrons/hardware_control/dev_types.py if possible typing_extensions should be imported in a failure-tolerant way and types dependent on it should be in string annotations and only checked statically.

If not possible to keep typing_extensions to only static use, we should add it to the buildroot config.

Traceback (reversed order from journalctl -r):

Mar 24 15:11:42 OT2CEP20191129B16 python[385]: ModuleNotFoundError: No module named 'typing_extensions'
Mar 24 15:11:42 OT2CEP20191129B16 python[385]:   File "usr/lib/python3.7/site-packages/robot_server/service/models/json_api/request.py", line 2, in <module>
Mar 24 15:11:42 OT2CEP20191129B16 python[385]:   File "usr/lib/python3.7/site-packages/robot_server/service/models/json_api/factory.py", line 3, in <module>
Mar 24 15:11:42 OT2CEP20191129B16 python[385]:   File "usr/lib/python3.7/site-packages/robot_server/service/routers/item.py", line 5, in <module>
Mar 24 15:11:42 OT2CEP20191129B16 python[385]:   File "usr/lib/python3.7/site-packages/robot_server/service/main.py", line 11, in <module>
Mar 24 15:11:42 OT2CEP20191129B16 python[385]:   File "usr/lib/python3.7/site-packages/robot_server/service/__init__.py", line 18, in run
Mar 24 15:11:42 OT2CEP20191129B16 python[385]:   File "usr/lib/python3.7/site-packages/robot_server/main.py", line 64, in run
Mar 24 15:11:42 OT2CEP20191129B16 python[385]:   File "usr/lib/python3.7/site-packages/robot_server/main.py", line 94, in main
Mar 24 15:11:42 OT2CEP20191129B16 python[385]:   File "usr/lib/python3.7/site-packages/robot_server/main.py", line 100, in <module>
Mar 24 15:11:42 OT2CEP20191129B16 python[385]:   File "/buildroot/output/target/usr/lib/python3.7/runpy.py", line 85, in _run_code
Mar 24 15:11:42 OT2CEP20191129B16 python[385]:   File "/buildroot/output/target/usr/lib/python3.7/runpy.py", line 193, in _run_module_as_main
Mar 24 15:11:42 OT2CEP20191129B16 python[385]: Traceback (most recent call last)