Cleanup: split BPY_extern_python into it's own header

Currently BPY_extern.h includes too much mixed functionality.

source/blender/python/BPY_extern_python.h

@@ -0,0 +1,40 @@
+/*
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ */
+
+/** \file
+ * \ingroup python
+ *
+ * Functionality relating to Python setup & tear down.
+ */
+
+#pragma once
+
+struct bContext;
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/* bpy_interface.c */
+void BPY_python_start(int argc, const char **argv);
+void BPY_python_end(void);
+void BPY_python_reset(struct bContext *C);
+void BPY_python_use_system_env(void);
+void BPY_python_backtrace(/* FILE */ void *file);
+
+#ifdef __cplusplus
+} /* extern "C" */
+#endif