fix test failures

src/pip/_internal/index/collector.py

@@ -8,7 +8,23 @@
 import mimetypes
 import os
 from collections import OrderedDict
-from functools import lru_cache
+
+try:
+    from functools import lru_cache
+except ImportError:
+    # Only works for single-argument functions.
+    def lru_cache(*args, **kwargs):
+        cache = {}
+        def wrapper(fn):
+            def wrapped(arg):
+                value = cache.get(arg, None)
+                if value is not None:
+                    return value
+                value = fn(arg)
+                cache[arg] = value
+                return value
+            return wrapped
+        return wrapper
 
 from pip._vendor import html5lib, requests
 from pip._vendor.distlib.compat import unescape

tests/unit/test_collector.py

@@ -290,12 +290,12 @@ def test_parse_links_caches_same_page():
     page_1 = HTMLPage(
         html_bytes,
         encoding=None,
-        url='https://example.com/simple/',
+        url='https://example.com/some-find-links-url/',
     )
     page_2 = HTMLPage(
         html_bytes,
         encoding=None,
-        url='https://example.com/simple/',
+        url='https://example.com/some-find-links-url/',
     )
 
     with mock.patch("pip._internal.index.collector.html5lib.parse") as mock_parse: