prc: Add `ConfigVariableFilename.__fspath__()`

Fixes #1406

dtool/src/prc/configVariableFilename.I

@@ -218,6 +218,17 @@ set_word(size_t n, const Filename &value) {
   set_string_word(n, value);
 }
 
+/**
+ * Allows a ConfigVariableFilename object to be passed to any Python function
+ * that accepts an os.PathLike object.
+ *
+ * @since 1.10.13
+ */
+INLINE std::wstring ConfigVariableFilename::
+__fspath__() const {
+  return get_ref_value().to_os_specific_w();
+}
+
 /**
  * Returns the variable's value, as a reference into the config variable
  * itself.  This is the internal method that implements get_value(), which

dtool/src/prc/configVariableFilename.h

@@ -60,6 +60,8 @@ PUBLISHED:
   INLINE Filename get_word(size_t n) const;
   INLINE void set_word(size_t n, const Filename &value);
 
+  INLINE std::wstring __fspath__() const;
+
 private:
   void reload_cache();
   INLINE const Filename &get_ref_value() const;