Test and fix for text attached to end XML elements (#18)

The ElementTree's `XMLPullParser` sometimes attaches the text to the
start element and sometimes to the end element. We could not figure out
why there is a different behavior, but suspect that it has something to
do with the size of the parser buffer.

In this patch, we fix by looking for text in both events (start as well
as end element).

This is related to the issue #17, and based on the aas-core-codegen
after the pull request
aas-core-works/aas-core-codegen#443 has been
merged in, [aas-core-codegen 330f391].

[aas-core-codegen 330f391]: aas-core-works/aas-core-codegen@330f391

aas_core3/xmlization.py

@@ -9340,7 +9340,7 @@ def _raise_if_has_tail_or_attrib(element: Element) -> None:
 
 def _read_end_element(
     element: Element, iterator: Iterator[Tuple[str, Element]]
-) -> None:
+) -> Element:
     """
     Read the end element corresponding to the start :paramref:`element`
     from :paramref:`iterator`.
@@ -9367,6 +9367,8 @@ def _read_end_element(
 
     _raise_if_has_tail_or_attrib(next_element)
 
+    return next_element
+
 
 def _read_text_from_element(
     element: Element, iterator: Iterator[Tuple[str, Element]]
@@ -9387,14 +9389,20 @@ def _read_text_from_element(
     """
     _raise_if_has_tail_or_attrib(element)
 
-    if element.text is None:
-        raise DeserializationException(
-            "Expected an element with text, but got an element with no text."
-        )
-
     text = element.text
 
-    _read_end_element(element, iterator)
+    end_element = _read_end_element(
+        element,
+        iterator,
+    )
+
+    if text is None:
+        if end_element.text is None:
+            raise DeserializationException(
+                "Expected an element with text, but got an element with no text."
+            )
+
+        text = end_element.text
 
     return text
 
@@ -9520,10 +9528,15 @@ def _read_str_from_element_text(
     # the ``element`` to contain *some* text. In contrast, this function
     # can also deal with empty text, in which case it returns an empty string.
 
-    _raise_if_has_tail_or_attrib(element)
-    result = element.text if element.text is not None else ""
+    text = element.text
 
-    _read_end_element(element, iterator)
+    end_element = _read_end_element(element, iterator)
+
+    if text is None:
+        text = end_element.text
+
+    _raise_if_has_tail_or_attrib(element)
+    result = text if text is not None else ""
 
     return result