fix: Replace default file_cache usage with simple memory cache

[aawilson]

The original main.py creates noisy log messages when the function is run file_cache is unavailable when using oauth2client >= 4.0.0:

2020-06-26T19:34:03.459Z datastore_export file_cache is unavailable when using oauth2client >= 4.0.0 or google-auth E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export Traceback (most recent call last): E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export   File "/env/local/lib/python3.7/site-packages/googleapiclient/discovery_cache/file_cache.py", line 33, in <module> E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export     from oauth2client.contrib.locked_file import LockedFile E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export ModuleNotFoundError: No module named 'oauth2client' E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export  E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export During handling of the above exception, another exception occurred: E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export  E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export Traceback (most recent call last): E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export   File "/env/local/lib/python3.7/site-packages/googleapiclient/discovery_cache/file_cache.py", line 37, in <module> E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export     from oauth2client.locked_file import LockedFile E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export ModuleNotFoundError: No module named 'oauth2client' E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export  E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export During handling of the above exception, another exception occurred: E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export  E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export Traceback (most recent call last): E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export   File "/env/local/lib/python3.7/site-packages/googleapiclient/discovery_cache/__init__.py", line 44, in autodetect E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export     from . import file_cache E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export   File "/env/local/lib/python3.7/site-packages/googleapiclient/discovery_cache/file_cache.py", line 41, in <module> E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export     "file_cache is unavailable when using oauth2client >= 4.0.0 or google-auth" E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export ImportError: file_cache is unavailable when using oauth2client >= 4.0.0 or google-auth E  datastore_export 

While they don't interfere with the function's operation, they are disconcerting and not necessary. This implements the workaround suggested in googleapis/google-api-python-client#325 (which creates an in-memory cache object to use in replacement).

Description

Fixes #4185

Checklist

• I have followed Sample Guidelines from AUTHORING_GUIDE.MD
• README is updated to include all relevant information
• Tests pass: nox -s py-3.6 (see Test Enviroment Setup)
• Lint pass: nox -s lint (see Test Enviroment Setup)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• Please merge this PR for me once it is approved.

[leahecole]

PTAL @BenWhitehead or @crwilcox to see how you feel about it from a Datastore point of view

[leahecole]

@aawilson it looks like this is failing lint - there needs to be one more blank line in main.py line 20. You can also test this locally with nox -s lint (see Test Enviroment Setup)

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[aawilson]

Yup got it, my local linter would have caught it but I've been editing this entirely in GitHub's interface, will add

[leahecole]

@aawilson I've been there! Thanks for the edits 😄