fix: Fix Jinja2 error for Jinja2 < 2.11

Versions of Jinja2 before 2.11 (or 2.11.1)
crash when a Path-like object is given
to the `FileSystemLoader` constructor.

We fix this simply by casting the Path object
to a string.

See issues #67 and #72.

src/mkdocstrings/handlers/__init__.py

@@ -123,7 +123,7 @@ def __init__(self, directory: str, theme: str) -> None:
             else:
                 raise ThemeNotSupported(theme)
 
-        self.env = Environment(autoescape=True, loader=FileSystemLoader(theme_dir))
+        self.env = Environment(autoescape=True, loader=FileSystemLoader(str(theme_dir)))
         self.env.filters["highlight"] = do_highlight
         self.env.filters["any"] = do_any