config: Use "mapping" instead of "dict" for user-facing errors

Previously, these errors used the word "dict" because it's the common
term in the Python world. But the YAML term is rather "mapping". The
present commit fixes this wording.

This originates from this discussion [^2].

[^1]: https://yaml.org/spec/1.2.2/#3211-nodes
[^2]: #747 (review)

Author: adrienverge

yamllint/config.py

@@ -79,11 +79,12 @@ def parse(self, raw_content):
             raise YamlLintConfigError(f'invalid config: {e}') from e
 
         if not isinstance(conf, dict):
-            raise YamlLintConfigError('invalid config: not a dict')
+            raise YamlLintConfigError('invalid config: not a mapping')
 
         self.rules = conf.get('rules', {})
         if not isinstance(self.rules, dict):
-            raise YamlLintConfigError('invalid config: rules should be a dict')
+            raise YamlLintConfigError('invalid config: rules should be a '
+                                      'mapping')
         for rule in self.rules:
             if self.rules[rule] == 'enable':
                 self.rules[rule] = {}
@@ -235,7 +236,7 @@ def validate_rule_conf(rule, conf):
     else:
         raise YamlLintConfigError(
             f'invalid config: rule "{rule.ID}": should be either "enable", '
-            f'"disable" or a dict')
+            f'"disable" or a mapping')
 
     return conf