Update dev guide, lock aiohttp to a version that works.

[dblock]

Description

Update developer guide with test setup instructions using OpenSearch 2.12+.

The two failing tests.

FAILED test_opensearchpy/test_async/test_http_connection.py::TestAsyncHttpConnection::test_basicauth_in_request_session - TypeError: 'coroutine' object is not iterable
FAILED test_opensearchpy/test_async/test_http_connection.py::TestAsyncHttpConnection::test_callable_in_request_session - TypeError: 'coroutine' object is not iterable

The headers in warning_headers = response.headers.getall("warning", ()) changed between aiohttp 3.9 and 3.10 from () to <coroutine object AsyncMockMixin._execute_mock_call at 0x103732ff0>, not sure why.

For now I opened aio-libs/aiohttp#9297 and locked the version to <= 3.10.5.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.34%. Comparing base (ba715b9) to head (863ff9b).
Report is 60 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #828      +/-   ##
==========================================
- Coverage   71.95%   70.34%   -1.61%     
==========================================
  Files          91      113      +22     
  Lines        8001     8875     +874     
==========================================
+ Hits         5757     6243     +486     
- Misses       2244     2632     +388     

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