Resolve #241 Support python >= 3.6

Notes:
In py36 the typing module is flattening baseclasses in
unions eg Union[int, bool] -> (int), there is no official
workaround but from all my samples I 've only encountered this case
in a few tests cases in the w3c test suite.

.travis.yml

@@ -3,6 +3,8 @@ cache: pip
 
 jobs:
   include:
+    - python: 3.6
+      env: TOXENV=py36
     - python: 3.7
       env: TOXENV=py37
     - python: 3.8

setup.cfg

@@ -16,6 +16,7 @@ classifiers =
     Programming Language :: Python
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3 :: Only
+    Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9
@@ -37,7 +38,7 @@ install_requires =
     lxml
     requests
     toposort
-python_requires = >=3.7
+python_requires = >=3.6
 include_package_data = True
 
 [options.entry_points]
@@ -52,6 +53,7 @@ dev =
     pytest-benchmark
     pytest-cov
     tox
+    dataclasses;python_version<"3.7"
 docs =
     sphinx
     sphinx-autobuild

tests/formats/dataclass/test_context.py

@@ -1,12 +1,16 @@
+import sys
 from dataclasses import dataclass
 from dataclasses import make_dataclass
 from dataclasses import replace
 from typing import get_type_hints
 from typing import Iterator
 from typing import List
+from typing import Union
 from unittest import mock
 from unittest import TestCase
 
+import pytest
+
 from tests.fixtures.books import BookForm
 from tests.fixtures.books import Books
 from tests.fixtures.defxmlschema.chapter03prod import Product
@@ -19,6 +23,7 @@
 from xsdata.formats.dataclass.models.elements import XmlAttribute
 from xsdata.formats.dataclass.models.elements import XmlElement
 from xsdata.formats.dataclass.models.elements import XmlMeta
+from xsdata.formats.dataclass.models.elements import XmlText
 from xsdata.formats.dataclass.models.elements import XmlWildcard
 from xsdata.utils import text
 
@@ -202,6 +207,23 @@ def test_get_type_hints(self):
             self.assertFalse(var.dataclass)
             self.assertIsNone(var.clazz)
 
+    @pytest.mark.skipif(sys.version_info < (3, 7), reason="requires python >= 3.7")
+    def test_get_type_hints_with_union_types(self):
+        @dataclass
+        class Example:
+            bool: bool
+            int: int
+            union: Union[int, bool]
+
+        result = list(self.ctx.get_type_hints(Example, None))
+        expected = [
+            XmlText(name="bool", qname="bool", types=[bool]),
+            XmlText(name="int", qname="int", types=[int]),
+            XmlText(name="union", qname="union", types=[bool, int]),
+        ]
+
+        self.assertEqual(expected, result)
+
     def test_get_type_hints_with_dataclass_list(self):
         result = list(self.ctx.get_type_hints(Books, None))
 

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py37,py38,py39
+envlist = py36,py37,py38,py39
 skip_missing_interpreters = true
 
 [testenv]

xsdata/formats/dataclass/context.py

@@ -238,12 +238,12 @@ def is_derived(cls, obj: Any, clazz: Type) -> bool:
 
     @staticmethod
     def is_element_list(type_hint: Any, is_tokens: bool) -> bool:
-        if getattr(type_hint, "__origin__", None) is list:
+        if getattr(type_hint, "__origin__", None) in (list, List):
             if not is_tokens:
                 return True
 
             type_hint = type_hint.__args__[0]
-            if getattr(type_hint, "__origin__", None) is list:
+            if getattr(type_hint, "__origin__", None) in (list, List):
                 return True
 
         return False