Fix configtypes.Perc.to_str()

If we used an int/float in config.py for a Perc value (e.g. zoom.default),
to_str() returned int/float instead of str, causing qWarnings and bugs.

qutebrowser/config/configtypes.py

@@ -847,7 +847,10 @@ class Perc(_Numeric):
     def to_str(self, value: typing.Union[None, float, int, str]) -> str:
         if value is None:
             return ''
-        return value
+        elif isinstance(value, str):
+            return value
+        else:
+            return '{}%'.format(value)
 
 
 class PercOrInt(_Numeric):

tests/unit/config/test_configtypes.py

@@ -1113,8 +1113,13 @@ class TestPerc:
         with pytest.raises(configexc.ValidationError):
             klass(**kwargs).to_py(val)
 
-    def test_to_str(self, klass):
-        assert klass().to_str('42%') == '42%'
+    @pytest.mark.parametrize('value, expected', [
+        ('42%', '42%'),
+        (42, '42%'),
+        (42.5, '42.5%'),
+    ])
+    def test_to_str(self, klass, value, expected):
+        assert klass().to_str(value) == expected
 
 
 class TestPercOrInt: