_API_DIGESTS in test_patches.py swallowed test cases

[jakob-keller]

Describe the bug
test_patches.py maintains a collection of upstream code that is patched by aiobotocore:

aiobotocore/tests/test_patches.py

Line 142 in d145aea

_API_DIGESTS = {

The patched objects are used as dictionary keys, which results in multiple test cases being swallowed due to hash collisions, e.g.:

aiobotocore/tests/test_patches.py

Lines 427 to 438 in d145aea

	RestXMLParser._create_event_stream: {
	'0564ba55383a71cc1ba3e5be7110549d7e9992f5'
	},
	EC2QueryParser._create_event_stream: {
	'0564ba55383a71cc1ba3e5be7110549d7e9992f5'
	},
	QueryParser._create_event_stream: {
	'0564ba55383a71cc1ba3e5be7110549d7e9992f5'
	},
	JSONParser._create_event_stream: {
	'0564ba55383a71cc1ba3e5be7110549d7e9992f5'
	},

Solution: Either those redundant entries are merged or the dict is replaced, i.e. by a tuple of tuples.

Checklist

• I have reproduced in environment where pip check passes without errors
• I have provided pip freeze results
• I have provided sample code or detailed way to reproduce
• I have tried the same code in botocore to ensure this is an aiobotocore specific issue
• I have tried similar code in aiohttp to ensure this is is an aiobotocore specific issue
• I have checked the latest and older versions of aiobotocore/aiohttp/python to see if this is a regression / injection

pip freeze results

Environment:

• Python Version: [e.g. 3.6]
• OS name and version: [e.g. darwin stretch]

Additional context
Add any other context about the problem here.