[3.6] bpo-31792: Restore os.environ in test_buffer when import numpy. (GH-4007) (#4009)

(cherry picked from commit 676db4bbf2)

Lib/test/test_buffer.py

@@ -17,7 +17,7 @@ from test import support
 from itertools import permutations, product
 from random import randrange, sample, choice
 import warnings
-import sys, array, io
+import sys, array, io, os
 from decimal import Decimal
 from fractions import Fraction
 
@@ -37,7 +37,8 @@ except ImportError:
     ctypes = None
 
 try:
-    with warnings.catch_warnings():
+    with support.EnvironmentVarGuard() as os.environ, \
+         warnings.catch_warnings():
         from numpy import ndarray as numpy_array
 except ImportError:
     numpy_array = None