MNT: use BytesIO contextmanager

Co-authored-by: Elliott Sales de Andrade <[email protected]>

Author: tacaswell

lib/matplotlib/tests/test_axes.py

@@ -8455,5 +8455,5 @@ def test_zorder_and_explicit_rasterization():
     fig, ax = plt.subplots()
     ax.set_rasterization_zorder(5)
     ln, = ax.plot(range(5), rasterized=True, zorder=1)
-    b = io.BytesIO()
-    fig.savefig(b, format='pdf')
+    with io.BytesIO() as b:
+        fig.savefig(b, format='pdf')