Support PathLike objects in base_path (#2206)

* Support PathLike objects in base_path

This enables this usage in MkDocs (note that specifying as a list already works):

```yml
  - pymdownx.snippets:
      base_path: !relative $docs_dir
```

Currently you get an error:
```
               self.base_path = [os.path.abspath(b) for b in base]
                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
           TypeError: 'DocsDirPlaceholder' object is not iterable
```

* Add test

pymdownx/snippets.py

@@ -80,7 +80,7 @@ def __init__(self, config, md):
         """Initialize."""
 
         base = config.get('base_path')
-        if isinstance(base, str):
+        if isinstance(base, (str, os.PathLike)):
             base = [base]
         self.base_path = [os.path.abspath(b) for b in base]
         self.restrict_base_path = config['restrict_base_path']

tests/test_extensions/test_snippets.py

@@ -454,6 +454,39 @@ def test_section_no_end(self):
         )
 
 
+class _PathLikeExampleObject:
+    def __fspath__(self):
+        return os.path.join(BASE, '_snippets')
+
+
+class TestSnippetsPathLike(util.MdCase):
+    """Test snippet cases with path-like objects."""
+
+    extension = [
+        'pymdownx.snippets'
+    ]
+
+    extension_configs = {
+        'pymdownx.snippets': {
+            'base_path': _PathLikeExampleObject()
+        }
+    }
+
+    def test_inline(self):
+        """Test inline."""
+
+        self.check_markdown(
+            R'''
+            ---8<--- "a.txt"
+            ''',
+            R'''
+            <p>Snippet</p>
+            ''',
+            True
+        )
+
+
+
 class TestSnippetsFile(util.MdCase):
     """Test snippet file case."""