Use PathLike feature detection for savefig param

Reworks the previous commit using pathlib.Path in favor of more
flexible os.PathLike detection.

The purpose of pathlib.Path is basically to provide an os.PathLike interface
for other libraries to read (via os.fspath()), but any object can also
provide the same interface. If we check for the interface directly,
the output options can be extended without adding more type checks.

os.PathLike is newer (added in 3.6) than pathlib.Path (3.4), but
it looks like mplfinance is already 3.6+, so version compat seems fine.

Author: mattsta

src/mplfinance/plotting.py

@@ -7,8 +7,8 @@
 import numpy  as np
 import copy
 import io
+import os
 import math
-import pathlib
 import warnings
 import statistics as stat
 
@@ -169,7 +169,7 @@ def _valid_plot_kwargs():
                                         'Validator'   : lambda value: isinstance(value,dict) or (isinstance(value,list) and all([isinstance(d,dict) for d in value])) },
 
         'savefig'                   : { 'Default'     : None, 
-                                        'Validator'   : lambda value: isinstance(value,dict) or isinstance(value,str) or isinstance(value, io.BytesIO) or isinstance(value, pathlib.Path) },
+                                        'Validator'   : lambda value: isinstance(value,dict) or isinstance(value,str) or isinstance(value, io.BytesIO) or isinstance(value, os.PathLike) },
 
         'block'                     : { 'Default'     : None, 
                                         'Validator'   : lambda value: isinstance(value,bool) },