functools.partial objects do not always have a __module__ attribute, …

[Limfocit]

Fixes #1232

When working with the latest version of the library, got an error

  1752         return plot(
   1753             results=results,
   1754             df=self._data,
   (...)
   1769             show_legend=show_legend,
   1770             open_browser=open_browser)
   1773 # NOTE: Don't put anything public below this __all__ list
   1775 __all__ = [getattr(v, '__name__', k)
   1776            for k, v in globals().items()                        # export
-> 1777            if ((callable(v) and v.__module__ == __name__ or     # callables from this module
   1778                 k.isupper()) and                                # or CONSTANTS
   1779                not getattr(v, '__name__', k).startswith('_'))]  # neither marked internal
   1781 # NOTE: Don't put anything public below here. See above.

AttributeError: 'functools.partial' object has no attribute '__module__'

The error is occurring because functools.partial objects do not always have a module attribute, but the backtesting.py library is assuming that all callable objects do.
With the suggested fix, the code started to work.