Replace force_text with force_str (#173)

aliases and force_text is deprecated with Django 3.0

Author: jannh

src/bootstrap4/text.py

@@ -1,14 +1,11 @@
-try:
-    from django.utils.encoding import force_text
-except ImportError:
-    from django.utils.encoding import force_unicode as force_text
+from django.utils.encoding import force_str
 
 
 def text_value(value):
     """Force a value to text, render None as an empty string."""
     if value is None:
         return ""
-    return force_text(value)
+    return force_str(value)
 
 
 def text_concat(*args, **kwargs):