perf: add a new "context" argument to timer

This allow to simple setup/teardown outside of the timed section. Especially
using object that need context manager, like a temporary files.

contrib/perf.py

@@ -532,10 +532,16 b' DEFAULTLIMITS = ('
 )
 
 
[e-mail]
+def noop_context():
+    yield
+
+
 def _timer(
     fm,
     func,
     setup=None,
+    context=noop_context,
     title=None,
     displayall=False,
     limits=DEFAULTLIMITS,
@@ -551,14 +557,16 b' def _timer('
     for i in range(prerun):
         if setup is not None:
             setup()
-        func()
+        with context():
+            func()
     keepgoing = True
     while keepgoing:
         if setup is not None:
             setup()
-        with profiler:
-            with timeone() as item:
-                r = func()
+        with context():
+            with profiler:
+                with timeone() as item:
+                    r = func()
         profiler = NOOPCTX
         count += 1
         results.append(item[0])