Default warnings behaviour

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

```
>>> import warnings
>>> warnings.simplefilter("default")
>>> import pandas
:914: ImportWarning: _SixMetaPathImporter.find_spec() not found; falling back to find_module()
:914: ImportWarning: _SixMetaPathImporter.find_spec() not found; falling back to find_module()
: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.