Use functools.lru_cache to cache supported tags for wheels.

This is a suggestion by @abravalheri for my PR.
#3805 (comment)

setuptools/tests/test_wheel.py

@@ -612,9 +612,17 @@ def sys_tags():
         for t in parse_tag('cp36-cp36m-manylinux1_x86_64'):
             yield t
     monkeypatch.setattr('setuptools.wheel.sys_tags', sys_tags)
-    monkeypatch.setattr('setuptools.wheel._supported_tags', None)
-    assert Wheel(
-        'onnxruntime-0.1.2-cp36-cp36m-manylinux1_x86_64.whl').is_compatible()
+    # Clear the supported tags cache, otherwise the sys_tags monkeypatch
+    # has no effect.
+    setuptools.wheel._supported_tags.cache_clear()
+    try:
+        assert Wheel(
+            'onnxruntime-0.1.2-cp36-cp36m-manylinux1_x86_64.whl'
+        ).is_compatible()
+    finally:
+        # Clear the cache again, otherwise the sys_tags monkeypatch
+        # is still in effect for the rest of the tests.
+        setuptools.wheel._supported_tags.cache_clear()
 
 
 def test_wheel_mode():

setuptools/wheel.py

@@ -2,6 +2,7 @@
 
 import email
 import itertools
+import functools
 import os
 import posixpath
 import re
@@ -27,7 +28,13 @@
 NAMESPACE_PACKAGE_INIT = \
     "__import__('pkg_resources').declare_namespace(__name__)\n"
 
-_supported_tags = None
+
+@functools.lru_cache(maxsize=None)
+def _get_supported_tags():
+    # We calculate the supported tags only once, otherwise calling
+    # this method on thousands of wheels takes seconds instead of
+    # milliseconds.
+    return set((t.interpreter, t.abi, t.platform) for t in sys_tags())
 
 
 def unpack(src_dir, dst_dir):
@@ -85,13 +92,7 @@ def tags(self):
 
     def is_compatible(self):
         '''Is the wheel compatible with the current platform?'''
-        global _supported_tags
-        if _supported_tags is None:
-            # We calculate the supported tags only once, otherwise calling
-            # this method on thousands of wheels takes seconds instead of
-            # milliseconds.
-            _supported_tags = set(
-                (t.interpreter, t.abi, t.platform) for t in sys_tags())
+        _supported_tags = _get_supported_tags()
         return next((True for t in self.tags() if t in _supported_tags), False)
 
     def egg_name(self):