return least complex marker in exclude (for consistency)

Otherwise, the expectations in the export tests had to be changed due to the previous commit.

src/poetry/core/version/markers.py

@@ -744,7 +744,7 @@ def exclude(self, marker_name: str) -> BaseMarker:
             if not marker.is_empty():
                 new_markers.append(marker)
 
-        return self.of(*new_markers)
+        return intersection(*new_markers)
 
     def only(self, *marker_names: str) -> BaseMarker:
         return self.of(*(m.only(*marker_names) for m in self._markers))
@@ -920,7 +920,7 @@ def exclude(self, marker_name: str) -> BaseMarker:
             # All markers were the excluded marker.
             return AnyMarker()
 
-        return self.of(*new_markers)
+        return union(*new_markers)
 
     def only(self, *marker_names: str) -> BaseMarker:
         return self.of(*(m.only(*marker_names) for m in self._markers))

tests/version/test_markers.py

@@ -1351,6 +1351,18 @@ def test_without_extras(marker: str, expected: str) -> None:
             "extra",
             'python_version >= "3.6"',
         ),
+        (
+            (
+                'python_version >= "2.7" and (python_version < "2.8"'
+                ' or python_version >= "3.7") and python_version < "3.8"'
+                ' and extra == "foo"'
+            ),
+            "extra",
+            (
+                'python_version >= "2.7" and python_version < "2.8"'
+                ' or python_version >= "3.7" and python_version < "3.8"'
+            ),
+        ),
     ],
 )
 def test_exclude(marker: str, excluded: str, expected: str) -> None: