fix: disable ruamel.yaml based backend if its lib is too old

Disable ruamel.yaml based YAML backend if dependent ruamel.yaml library
is too old and does not have the definition of ruamel.yaml.YAML class,
necessary to make it works.

This fix was based on the report from the pr#97 by Masatake YAMATO.

anyconfig/backend/yaml/ruamel_yaml.py

@@ -44,6 +44,11 @@
 from . import pyyaml
 
 
+try:
+    ryaml.YAML  # flake8: noqa
+except AttributeError:
+    ImportError("ruamel.yaml may be too old to use!")
+
 _YAML_INIT_KWARGS = ["typ", "pure", "plug_ins"]  # kwargs for ruamel.yaml.YAML
 _YAML_INSTANCE_MEMBERS = ['allow_duplicate_keys', 'allow_unicode',
                           'block_seq_indent', 'canonical', 'composer',