Update PyYAML to version 5.1

Update use of safe_load and safe_dump to load and dump using
the CSafeLoader with SafeLoader as a fallback.

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

mayan/apps/ocr/runtime.py

@@ -2,6 +2,11 @@ from __future__ import unicode_literals
 
 import yaml
 
+try:
+    from yaml import CSafeLoader as SafeLoader
+except ImportError:
+    from yaml import SafeLoader
+
 from django.utils.module_loading import import_string
 
 from .settings import setting_ocr_backend, setting_ocr_backend_arguments
@@ -9,7 +14,7 @@ from .settings import setting_ocr_backend, setting_ocr_backend_arguments
 ocr_backend = import_string(
     setting_ocr_backend.value
 )(
-    **yaml.safe_load(
-        setting_ocr_backend_arguments.value or '{}'
+    **yaml.load(
+        stream=setting_ocr_backend_arguments.value or '{}', Loader=SafeLoader
     )
 )