types: add option for strict connection types

[shrouxm]

This PR adds a strict_types meta option to the relay Connection class to simplify the use case identified in this issue: graphql-python/graphene-django#901

Default of False is the old behavior, and when set to True generated TConnection types will be typed as edges: [TEdge!]! instead of edges: [TEdge]! and TEdge will have node: T! instead of node: T.

This avoids duplicating logic and string descriptions for subclasses which just want to implement this common behavior. If this behavior is deemed desirable by default it would be trivial to flip the flag's default or remove it entirely. It only make the types stricter so it oughtn't break any existing code (though it will cause unexpected changes to user's schema's nonetheless, which is why I kept the default to False).

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (6b8cd2d) 96.01% compared to head (035f19c) 96.01%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1504   +/-   ##
=======================================
  Coverage   96.01%   96.01%           
=======================================
  Files          51       51           
  Lines        1755     1755           
=======================================
  Hits         1685     1685           
  Misses         70       70           

Impacted Files	Coverage Δ
graphene/relay/connection.py	100.00% <100.00%> (ø)

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[erikwrede]

Thanks for the PR! Can you please add some test cases for this, testing the schema generation? 😊

[shrouxm]

@erikwrede does that look like what you meant?

[shrouxm]

@erikwrede it looks like the checks will never complete because the python 3.6 test workflow no longer exists after https://github.com/graphql-python/graphene/pull/1507/files but the GitHub repo settings still require that workflow to run