Vendor DataLoader from aiodataloader and move get_event_loop() out of __init__ function.

[flipbit03]

DataLoaders are trying to get the asyncio's event loop too early (on __init__) and this is generating problems like making pytest + async unusable. Since there is no event loop at that moment, the first DataLoader instantiated is actually creating a new event loop. This generates the infamous "Future tied to a different loop" problem many people are facing.

In this PR, we are:

• Vendoring aiodataloader's DataLoader class into the utils/ module
• Adding a fix that moves get_event_loop out of the class' initializer and into a property which only gets called when actually needed, offsetting the chicken-and-egg problem we are dealing with.

The base for this work is dataloader.py, which is MIT Licensed code from Syrus Akbary

[codecov]

Codecov Report

Merging #1459 (1e7bfb7) into master (45986b1) will increase coverage by 0.27%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1459      +/-   ##
==========================================
+ Coverage   95.44%   95.72%   +0.27%     
==========================================
  Files          49       50       +1     
  Lines        1582     1683     +101     
==========================================
+ Hits         1510     1611     +101     
  Misses         72       72              

Impacted Files	Coverage Δ
graphene/utils/dataloader.py	100.00% <100.00%> (ø)

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[flipbit03]

without this line, flake8 was constantly clashing with black over array slice notation whitespacing rules.

[erikwrede]

Seems like test_dataloader is never run, explaining the decrease in coverage