Some more fixes for the Python module type hinting.

[bcoconni]

Fixes a few errors that were remaining after the PR #1393.

• The stub file is renamed from _jsbsim.pyi to __init__.pyi so that it applies correctly to the module jsbsim instead of currently jsbsim._jsbsim.
• Also fixes a pair of type hintings where the transition from SGPropertyNode to FGPropertyNode (see PR Remove the class FGPropertyNode #1270) has been overlooked.
• Remove the useless statement from . import _jsbsim.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 25.12%. Comparing base (6c0e656) to head (d5ac5e6).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1415   +/-   ##
=======================================
  Coverage   25.12%   25.12%           
=======================================
  Files         169      169           
  Lines       18721    18721           
=======================================
  Hits         4703     4703           
  Misses      14018    14018           

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[agodemar]

OK to merge!