Default warnings behaviour

[alexmojaki]

friendly-traceback filters all warnings - see friendly-traceback/friendly-traceback#175. In some ways this is a good thing, e.g:

>>> import warnings
>>> warnings.simplefilter("default")
>>> import pandas
<frozen importlib._bootstrap>:914: ImportWarning: _SixMetaPathImporter.find_spec() not found; falling back to find_module()
<frozen importlib._bootstrap>:914: ImportWarning: _SixMetaPathImporter.find_spec() not found; falling back to find_module()
<frozen importlib._bootstrap>:914: ImportWarning: _SixMetaPathImporter.find_spec() not found; falling back to find_module()
>>> []()
/my_program_5.py:1: SyntaxWarning: 'list' object is not callable; perhaps you missed a comma?
  []()
/my_program_5.py:1: SyntaxWarning: 'list' object is not callable; perhaps you missed a comma?
  []()
>>> 

Ideally the behaviour should match the standard so that the warnings module works as expected and users can see potentially useful warnings, but at the same time this needs to be done in a way that avoids spurious noise and duplicates like the above.