title: CSSTransition does not work. V2 react-transition-group
body: <p>So, I have a tiny part of code where I'm trying to add an animation as wrapper for <code>Tooltip Nodes</code>. But perhaps I do something wrong, because I do not see on the screen any <code>animation</code> appearing during <code>mount</code>.</p>
  
  <p>Moreover, it even does not fire <code>console.log</code> on <code>onEnter</code> event. Why?</p>
  
  <p>Thanks.</p>
  
  <pre><code>import React, { PureComponent } from 'react'
  import { CSSTransition } from 'react-transition-group'
  
  import styles from './index.scss'
  import './anim.scss'
  
  class ToolTip extends PureComponent {
    render() {
      return (
          &lt;CSSTransition
            in={true}
            classNames={'tooltip'}
            timeout={3000}
            onEnter={() =&gt; { console.log('FIRED!') }}
          &gt;
            &lt;div className={`${styles.tooltipContainer}`}&gt;
              &lt;span className={styles.title}&gt;{'title'}&lt;/span&gt;
            &lt;/div&gt;
          &lt;/CSSTransition&gt;
      )
    }
  }
  
  export default ToolTip
  </code></pre>
  
  <p>Edit:</p>
  
  <p>my <code>anim.scss</code> file:</p>
  
  <pre><code>.tooltip-enter {
    opacity: 0;
  
    &amp;.tooltip-enter-active {
      opacity: 1;
    }
  }
  
  .tooltip-exit {
    opacity: 1;
  
    &amp;.tooltip-exit-active {
      opacity: 0;
    }
  }
  </code></pre>