Avoid extra copy initializing empty Affine2D

This saves a small amount of time each:
```
In [1]: from matplotlib.transforms import Affine2D
In [2]: %timeit Affine2D()
1.42 µs ± 6.17 ns per loop (mean ± std. dev. of 7 runs, 1,000,000 loops each)
```
vs
```
In [1]: from matplotlib.transforms import Affine2D
In [2]: %timeit Affine2D()
1.1 µs ± 13 ns per loop (mean ± std. dev. of 7 runs, 1,000,000 loops each)
In [3]: 1.1/1.42
Out[3]: 0.7746478873239437
```

Author: QuLogic

lib/matplotlib/transforms.py

@@ -1890,7 +1890,7 @@ def __init__(self, matrix=None, **kwargs):
         super().__init__(**kwargs)
         if matrix is None:
             # A bit faster than np.identity(3).
-            matrix = IdentityTransform._mtx.copy()
+            matrix = IdentityTransform._mtx
         self._mtx = matrix.copy()
         self._invalid = 0