fix: Replace default file_cache usage with simple memory cache (#4184)

* fix: Replace default file_cache usage with simple memory cache

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).

* Add explanatory comment

Add a comment explaining the inclusion of MemoryCache instance in the `build` invocation.

* Update main.py

PEP8-mandated two newlines between top-level elements

Co-authored-by: Christopher Wilcox <crwilcox@google.com>
Co-authored-by: Leah E. Cole <6719667+leahecole@users.noreply.github.com>

datastore/schedule-export/main.py

@@ -3,8 +3,22 @@
 import os
 
 from googleapiclient.discovery import build
+from googleapiclient.discovery_cache.base import Cache
 
-datastore = build('datastore', 'v1')
+
+class MemoryCache(Cache):
+    _CACHE = {}
+
+    def get(self, url):
+        return MemoryCache._CACHE.get(url)
+
+    def set(self, url, content):
+        MemoryCache._CACHE[url] = content
+
+
+# The default cache (file_cache) is unavailable when using oauth2client >= 4.0.0 or google-auth,
+# and it will log worrisome messages unless given another interface to use.
+datastore = build('datastore', 'v1', cache=MemoryCache())
 project_id = os.environ.get('GCP_PROJECT')