Bump apispec to the latest version (5.1.1)

[kiforchuk]

Fixes multiple issues related to apispec: #110, #105, #74

Relevant apispec changes:

• required keyword argument "location" in schema2parameters(): introduced in 4.0.0b1
• removed "format" from the docs: introduced in 4.0.0b1
• components have public attributes instead of the private ones: introduced in 4.2.0
• "allOf" is resolved now: introduced in 5.0.0

The following changes were done to aiohttp-apispec:

• bump apispec>=5.1.1
• remove python 3.5 support since the latest apispec doesn't support it
• pass appropriate location to schema2parameters() for the requests
• use "json" as a default location for request instead of "body" (deprecated in parser)
• validate schema location for the requests
• pass DEFAULT_RESPONSE_LOCATION="json" to schema2parameters() for the responses
• use "schemas" components public attribute instead of a private one
• remove "format" from tests
• other minor fixes: additional empty lines, fix deprecation warnings from marshmallow (pass description in metadata), add ref for "allOf", pass location as a string in tests

[kiforchuk]

@maximdanilchenko Your review will be highly appreciated
BTW isort seems to be broken :( can fix it in the next PR