special case union of single marker with inverse

To avoid ending up with constraints like:

bar==7.8.9; platform_system != 'Windows' or platform_system == 'Windows'

src/poetry/core/version/markers.py

@@ -284,6 +284,9 @@ def union(self, other: MarkerTypes) -> MarkerTypes:
             if self == other:
                 return self
 
+            if self == other.invert():
+                return AnyMarker()
+
             return MarkerUnion.of(self, other)
 
         return other.union(self)

tests/version/test_markers.py

@@ -240,6 +240,12 @@ def test_single_marker_union_with_union_duplicate():
     assert str(union) == 'sys_platform == "darwin" or python_version <= "3.6"'
 
 
+def test_single_marker_union_with_inverse():
+    m = parse_marker('sys_platform == "darwin"')
+    union = m.union(parse_marker('sys_platform != "darwin"'))
+    assert union.is_any()
+
+
 def test_multi_marker():
     m = parse_marker('sys_platform == "darwin" and implementation_name == "cpython"')