Schema output doesn't show if the field is non-nullable

[xmnlab]

The current schema doesn't show if the field is non-nullable

>>> ibis.schema([
...     ('foo', 'int64'),
...     ('bar', ibis.expr.datatypes.int64(nullable=False)),
...     ('baz', 'boolean*')
... ])

ibis.Schema {  
  foo  int64
  bar  int64
  baz  boolean
}

Desired behavior:

The output should show if the type is non-nullable, eg:

ibis.Schema {  
  foo  int64
  bar  int64[non-nullable]
  baz  boolean
}

[xmnlab]

suggestion: change DataType.__str__ method:

diff --git a/ibis/expr/datatypes.py b/ibis/expr/datatypes.py
index 9f469fb9..ec9998c9 100644
--- a/ibis/expr/datatypes.py
+++ b/ibis/expr/datatypes.py
@@ -83,7 +83,10 @@ class DataType:
         )
 
     def __str__(self) -> str:
-        return self.name.lower()
+        return '{}{}'.format(
+            self.name.lower(),
+            '[non-nullable]' if not self.nullable else ''
+        )
 
     @property
     def name(self) -> str: