More complex handling of open_browser from extension applications

[afshin]

This changes the default value open_browser in extension applications slightly.

Before this was set to True always. That means, even if ServerApp.open_browser=False, a browser was still opened. Now, the extension first checks if the server's open_browser trait was configured. If so, it uses this configured value; otherwise, it defaults to True.

In the case where both the starting extension application and the underlying server application receive a configuration value for open_browser, the extension application wins as you'd expect.

This PR also drops the --pyargs config from pytest.ini because it prevents you from running individual tests (always runs the whole test suite). Instead, it is advised to run pytest <path to tests>. Updated the README and GA workflows to reflect this change.

[codecov-io]

Codecov Report

Merging #433 (d18194f) into master (d07853e) will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #433      +/-   ##
==========================================
+ Coverage   77.48%   77.61%   +0.12%     
==========================================
  Files         105      105              
  Lines        8942     8964      +22     
  Branches      956      956              
==========================================
+ Hits         6929     6957      +28     
+ Misses       1676     1667       -9     
- Partials      337      340       +3     

Impacted Files	Coverage Δ
jupyter_server/extension/application.py	72.36% <100.00%> (-0.09%)	⬇️
jupyter_server/pytest_plugin.py	91.71% <100.00%> (+0.18%)	⬆️
jupyter_server/tests/extension/conftest.py	100.00% <100.00%> (ø)
...pyter_server/tests/extension/mockextensions/app.py	89.65% <100.00%> (+1.19%)	⬆️
jupyter_server/tests/extension/test_app.py	100.00% <100.00%> (ø)
jupyter_server/serverapp.py	66.62% <0.00%> (+0.77%)	⬆️

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[Zsailer]

Thanks, @afshin