fix .// xpath queries

Previously, when iter() encountered a non-Element child (like a text string), it would yield it unconditionally, even if the caller was searching for a specific tag (e.g., .//c).

```
    def test_Element_findall_dotslashslash():
        c1 = Element('c')
        c2 = Element('c')
        text = "text"
        b1 = Element('b', children=(c1, text, c2))
        b2 = Element('b')
        a1 = Element('a', children=(b1, b2, ))

        result = list(a1.findall('.//c'))
>       assert len(result) == 2
E       AssertionError: assert 3 == 2
E        +  where 3 = len([<Element 'c' at 1056c4450>, 'text', <Element 'c' at 1056c4810>])

src/emeraldtree/tests/test_tree.py:208: AssertionError
```

The fix ensures that non-Element children are only yielded if no tag filter is specified (tag is None).

Author: ThomasWaldmann

src/emeraldtree/tests/test_tree.py

@@ -184,7 +184,6 @@ def test_Element_findall_dotdot():
     assert result[1] is c2
 
 def test_Element_findall_slashslash():
-    pytest.skip('broken')
     c1 = Element('c')
     c2 = Element('c')
     text = "text"
@@ -199,7 +198,6 @@ def test_Element_findall_slashslash():
     assert result[1] is c2
 
 def test_Element_findall_dotslashslash():
-    pytest.skip('broken')
     c1 = Element('c')
     c2 = Element('c')
     text = "text"

src/emeraldtree/tree.py

@@ -355,7 +355,8 @@ def iter(self, tag=None):
                 for e in e.iter(tag):
                     yield e
             else:
-                yield e
+                if tag is None:
+                    yield e
 
     ##
     # Creates a text iterator.  The iterator loops over this element