simplify form code

app/auth/forms.py

@@ -23,7 +23,7 @@ class RegistrationForm(FlaskForm):
     username = StringField('Username',
         validators=[
             InputRequired(),
-            Length(min=1, max=64),
+            Length(1, 64),
             Regexp(
                 USERNAME_REGEX,
                 message='Usernames must have only letters, numbers, dots or underscores'  # noqa