Remove recommandations for unmaintained flex library

Fixes #285

src/drf_yasg/codecs.py

@@ -15,8 +15,12 @@ logger = logging.getLogger(__name__)
 
 
 def _validate_flex(spec):
-    from flex.core import parse as validate_flex
-    from flex.exceptions import ValidationError
+    try:
+        from flex.core import parse as validate_flex
+        from flex.exceptions import ValidationError
+    except ImportError:
+        return
+
     try:
         validate_flex(spec)
     except ValidationError as ex:

src/drf_yasg/views.py

@@ -57,7 +57,7 @@ def get_schema_view(info=None, url=None, patterns=None, urlconf=None, public=Fal
     :param patterns: same as :class:`.OpenAPISchemaGenerator`
     :param urlconf: same as :class:`.OpenAPISchemaGenerator`
     :param bool public: if False, includes only the endpoints that are accesible by the user viewing the schema
-    :param list validators: a list of validator names to apply; allowed values are ``flex``, ``ssv``
+    :param list validators: a list of validator names to apply; the only allowed value is ``ssv``, for now
     :param type generator_class: schema generator class to use; should be a subclass of :class:`.OpenAPISchemaGenerator`
     :param tuple authentication_classes: authentication classes for the schema view itself
     :param tuple permission_classes: permission classes for the schema view itself