adding email validator

running flask admin raised the following issue

`Exception: Install 'email_validator' for email validation support.`

examples/sqla/requirements.txt

@@ -7,6 +7,7 @@ enum34; python_version < '3.0'
 sqlalchemy_utils
 arrow
 colour
+email_validator
 
 # note: for local development, replace 'Flask-Admin' above with a reference to
 # your local copy of the repo e.g. '-e .' if you're installing this from the