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,
@@ -551,11 +557,13 b' def _timer('
     for i in range(prerun):
         if setup is not None:
             setup()
+        with context():
         func()
     keepgoing = True
     while keepgoing:
         if setup is not None:
             setup()
+        with context():
         with profiler:
             with timeone() as item:
                 r = func()