Fix: Do not use numeric tolerances for axline special cases

vertical lines (infinite slope) and two identical points as input need special handling in AxLine. The detection was using numeric tolerances, which lead to false-positive detection in cases that are close to but not exactly those special cases.

This PR removes the tolerances. The argument is the same as for the similar case matplotlib#28386 (comment)

Closes matplotlib#28870.

Author: timhoffm

lib/matplotlib/lines.py

@@ -1506,8 +1506,8 @@ def get_transform(self):
                 points_transform.transform([self._xy1, self._xy2])
             dx = x2 - x1
             dy = y2 - y1
-            if np.allclose(x1, x2):
-                if np.allclose(y1, y2):
+            if dx == 0:
+                if dy == 0:
                     raise ValueError(
                         f"Cannot draw a line through two identical points "
                         f"(x={(x1, x2)}, y={(y1, y2)})")