Avoid importing a non-vendored version of Tornado

Code depending on this conditional import could break if an old
version of Tornado is present in the environment, rendering pip
unusable.

Fixes pypa#10020

news/10020.bugfix.rst

@@ -0,0 +1 @@
+The presence of an old version of Tornado (before 4.1) in the environment no longer breaks pip. 

src/pip/_vendor/tenacity/__init__.py

@@ -23,7 +23,9 @@
     iscoroutinefunction = None
 
 try:
-    import tornado
+    # Pip does not currently vendor Tornado, so this will take the
+    # ImportError path.
+    from pip._vendor import tornado
 except ImportError:
     tornado = None
 

tools/vendoring/patches/tenacity.patch

@@ -0,0 +1,16 @@
+diff --git a/src/pip/_vendor/tenacity/__init__.py b/src/pip/_vendor/tenacity/__init__.py
+index 5f8cb50..571d2a3 100644
+--- a/src/pip/_vendor/tenacity/__init__.py
++++ b/src/pip/_vendor/tenacity/__init__.py
+@@ -23,7 +23,9 @@ except ImportError:
+     iscoroutinefunction = None
+
+ try:
+-    import tornado
++    # Pip does not currently vendor Tornado, so this will take the
++    # ImportError path.
++    from pip._vendor import tornado
+ except ImportError:
+     tornado = None
+
+