chore Fix docs sphinx nitpick warnings - adding intersphinx_mapping

[leszekhanusz]

Should fix #501

Note: some class references are still not found because the name of the class computed by sphinx locally does not correspond to the name in the objects.inv file generated by the upstream sphinx documentation.

Package	local name	upstream name (in objects.inv)
asyncio	asyncio.locks.Event	asyncio.Event
aiohttp	aiohttp.client_reqrep.Fingerprint	aiohttp.Fingerprint
aiohttp	aiohttp.helpers.BasicAuth	aiohttp.BasicAuth
graphql	graphql.execution.execute.ExecutionResult	graphql.execution.ExecutionResult
graphql	Source	graphql.language.Source or graphql.language.source.Source
graphql	GraphQLSchema	graphql.type.GraphQLSchema or graphql.type.schema.GraphQLSchema
multidict	multidict._multidict.CIMultiDictProxy	multidict.CIMultiDictProxy
multidict	multidict._multidict.CIMultiDict	multidict.CIMultiDict
multidict	multidict._multidict.istr	multidict.istr

They've been ignored in the sphinx docs with nitpick_ignore but we should fix this.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (57ef910) to head (a78ece9).
Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #515   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           30        30           
  Lines         3102      3127   +25     
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+ Hits          3102      3127   +25     

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