py/makeversionhdr.py: Work with backslashes in paths.

This script may be called by Windows IDEs (e.g. Visual Studio) and be passed
paths with backslashes.

py/makeversionhdr.py

@@ -57,7 +57,7 @@ def get_version_info_from_git():
     return git_tag, git_hash, ver
 
 def get_version_info_from_docs_conf():
-    with open("%s/docs/conf.py" % sys.argv[0].rsplit("/", 2)[0]) as f:
+    with open(os.path.join(os.path.dirname(sys.argv[0]), "..", "docs", "conf.py")) as f:
         for line in f:
             if line.startswith("release = '"):
                 ver = line.strip()[10:].strip("'")