Permanent reconnecting async session

[leszekhanusz]

New interface to allow to create a permanent reconnecting session which can be kept in a class.

# Create a session from the client which will reconnect automatically.
# This session can be kept in a class for example to provide a way
# to execute GraphQL queries from many different places
session = await client.connect_async(reconnecting=True)

# You can run execute or subscribe method on this session
result = await session.execute(query)

# When you want the connection to close (for cleanup),
# you call close_async
await client.close_async()

Should fix #179

[codecov-commenter]

Codecov Report

Merging #324 (2db7f5b) into master (6c91bb5) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #324   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           26        26           
  Lines         2159      2231   +72     
=========================================
+ Hits          2159      2231   +72     

Impacted Files	Coverage Δ
gql/client.py	100.00% <100.00%> (ø)
gql/transport/aiohttp.py	100.00% <100.00%> (ø)

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