Update font for linux

Author: goanpeca

colosseum/constants.py

@@ -335,7 +335,10 @@ def available_font_families():
     """List available font family names."""
     if sys.platform == 'darwin':
         return _available_font_families_mac()
-    return ['Arial Black']  # TODO: for tests
+    elif sys.platform.startswith('linux'):
+        return _available_font_families_unix()
+
+    return []  # TODO: for tests
 
 
 def _available_font_families_mac():
@@ -351,6 +354,14 @@ def _available_font_families_mac():
     return list(sorted(str(item) for item in manager.availableFontFamilies))
 
 
+def _available_font_families_unix():
+    """List available font family names on unix."""
+    import subprocess
+    p = subprocess.check_output(['fc-list', ':', 'family'])
+    fonts = p.decode().split('\n')
+    return list(sorted(set(fonts)))
+
+
 AVAILABLE_FONT_FAMILIES = available_font_families()
 FONT_FAMILY_CHOICES = Choices(
     validators=[is_font_family(generic_family=GENERIC_FAMILY_FONTS, font_families=AVAILABLE_FONT_FAMILIES)],