title: Python3 - "NameError: ... " when importing from Experta
body: <p>Hello,<br>
  I installed Experta in a Conda environment using Python 3.8.3 and all my programs work fine, but when I try to run this code importing Experta libraries I am having the following error: "NameError: name 'Fact' is not defined".</p>
  
  <pre><code>from random import choice
  from experta import *
  
  
  class Light(Fact):
      """Info about the traffic light."""
      pass
  
  
  class RobotCrossStreet(KnowledgeEngine):
      @Rule(Light(color='green'))
      def green_light(self):
          print("Walk")
  
      @Rule(Light(color='red'))
      def red_light(self):
          print("Don't walk")
  
      @Rule(AS.light &lt;&lt; Light(color=L('yellow') | L('blinking-yellow')))
      def cautious(self, light):
          print("Be cautious because light is", light["color"])
  
  
  engine = RobotCrossStreet()
  engine.reset()
  engine.declare(Light(color=choice(['green', 'yellow', 'blinking-yellow', 'red'])))
  engine.run()
  </code></pre>
  
  <p>Error:</p>
  
  <pre><code>Traceback (most recent call last):
  File "experta.py", line 2, in &lt;module&gt;
    from experta import *
  File "/home/karuro/Documents/Magierin/UPN/IntelligentSystems/Experta/experta.py", line 5, in &lt;module&gt;
    class Light(Fact): 
  NameError: name 'Fact' is not defined
  </code></pre>
  
  <p>Can anyone help me fix this?</p>