Allow for use_user_site being set to an integer

This can come from tox config, see:
pypa#7002 (comment)

src/pip/_internal/commands/install.py

@@ -616,11 +616,13 @@ def decide_user_install(
     If use_user_site is None, the default behaviour depends on the environment,
     which is provided by the other arguments.
     """
-    if use_user_site is False:
+    # In some cases (config from tox), use_user_site can be set to an integer
+    # rather than a bool. Comparing == True/False instead of 'is' allows this.
+    if use_user_site == False:  # noqa: E712
         logger.debug("Non-user install by explicit request")
         return False
 
-    if use_user_site is True:
+    if use_user_site == True:  # noqa: E712
         if prefix_path:
             raise CommandError(
                 "Can not combine '--user' and '--prefix' as they imply "