title: Unable to assign value '<unknown>' to attribute 'XXX'
body: <p>I'm trying to deploy the project to the App Engine but I'm getting the following error if I use the 'ü' character as a value inside the app.yaml. I don't want to HTML/URL encode the value. Is there any way to overcome this problem?</p>
  
  <pre><code>ERROR: (gcloud.app.deploy) An error occurred while parsing file: [app.yaml]
  Unable to assign value '&lt;unknown&gt;' to attribute 'XXX':
  Value u'\xfc' for XXX could not be converted to type str.
  </code></pre>
  
  <p>app.yaml file:</p>
  
  <pre><code>runtime: php72
  env: standard
  
  env_variables: 
    XXX: "ü"
  </code></pre>