Lock starlette requirement at <1.0

[rafalp]

Changes starlette requirement in setup.py to <1.0 so Ariadne will not complain about unsupported Starlette's version.

We are using Starlette mostly for handling ASGI requests, so 🤞 future version will not break our use case.

Fixes #956

[codecov]

Codecov Report

Merging #961 (8406883) into master (7a19ee1) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #961      +/-   ##
==========================================
- Coverage   98.13%   98.10%   -0.04%     
==========================================
  Files         109      111       +2     
  Lines        6716     6742      +26     
==========================================
+ Hits         6591     6614      +23     
- Misses        125      128       +3     

Impacted Files	Coverage Δ
tests/asgi/test_query_execution.py	97.77% <ø> (-1.12%)	⬇️
tests/wsgi/snapshots/snap_test_explorer.py	100.00% <ø> (ø)
ariadne/explorer/__init__.py	100.00% <100.00%> (ø)
ariadne/explorer/apollo.py	100.00% <100.00%> (ø)
ariadne/explorer/default_query.py	100.00% <100.00%> (ø)
ariadne/explorer/graphiql.py	100.00% <100.00%> (ø)
tests/asgi/snapshots/snap_test_explorer.py	100.00% <100.00%> (ø)
tests/asgi/test_http_methods.py	100.00% <100.00%> (ø)
tests/asgi/test_request_data_reading.py	100.00% <100.00%> (ø)
tests/explorers/snapshots/snap_test_explorers.py	100.00% <100.00%> (ø)
... and 3 more

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