python: handle Windows paths in trade.AbstractSceneConverter.begin_file().

Consistently with convert_to_file() and other APIs elsewhere.

doc/python/magnum.trade.rst

@@ -523,6 +523,11 @@
 .. py:function:: magnum.trade.AbstractSceneConverter.begin_file
     :raise RuntimeError: If beginning the conversion fails
 
+    For compatibility with :ref:`os.path`, on Windows this function converts
+    all backslashes in :p:`filename` to forward slashes before passing it to
+    :dox:`Trade::AbstractSceneConverter::beginFike()`, which expects forward
+    slashes as directory separators on all platforms.
+
 .. py:function:: magnum.trade.AbstractSceneConverter.end_file
     :raise AssertionError: If no conversion is in progress
     :raise RuntimeError: If ending the conversion fails

src/python/magnum/trade.cpp

@@ -1712,7 +1712,16 @@ void trade(py::module_& m) {
         /** @todo begin/end (MeshOptimizer), begin/end data */
         /** @todo drop std::string in favor of our own string caster */
         .def("begin_file", [](Trade::AbstractSceneConverter& self, const std::string& filename) {
-            if(!self.beginFile(filename)) {
+            if(!self.beginFile(
+                #ifdef CORRADE_TARGET_WINDOWS
+                /* To allow people to conveniently use Python's os.path, we
+                   need to convert backslashes to forward slashes as all
+                   Corrade and Magnum APIs expect forward */
+                Utility::Path::fromNativeSeparators(filename)
+                #else
+                filename
+                #endif
+            )) {
                 PyErr_SetString(PyExc_RuntimeError, "beginning the conversion failed");
                 throw py::error_already_set{};
             }