Use the 'htmlmin' python module, instead of 'django-htmlmin'

Signed-off-by: Mattia Rizzolo <mapreri@ubuntu.com>

setup.py

@@ -39,7 +39,7 @@ setuptools.setup(
         "pyyaml",
         "requests",
         "toml",
-        "django-htmlmin",
+        "htmlmin",
     ],
 
     packages = [

uitwww/utils.py

@@ -22,6 +22,14 @@ from htmlmin import minify as html_minify
 import gunicorn.app.base as baseapp
 
 
+# check that the html_minify we imported is a real function
+# (i.e., no the html_minify that comes from django-htmlmin)
+try:
+    assert callable(html_minify)
+except AssertionError:
+    raise ImportError("html_minify is not callable")
+
+
 def random_key(length):
     """Generate a random key of a given length"""
     rng = random.SystemRandom()  # Use /dev/urandom