Revert "Fix an error caused by a bug in Python < 2.7.9"

This reverts commit 0c3906f4dc.

Reason for revert: <broken compatibility with Python 3>

Original change's description:
> Fix an error caused by a bug in Python < 2.7.9
> 
> There seems to be a bug in Python versions prior
> to 2.7.9 where running exec could produce the following error:
> 
> SyntaxError: unqualified exec is not allowed in function
> '_ParsePythonTestTemplates' it contains a nested function
> with free variables (testcfg.py, line 71)
> 
> https://bugs.python.org/issue21591
> 
> It's causing an issue on all Ubuntu 14 and RHEL 7 machines.
> 
> The proposed change is an equivalent syntax which doesn't
> produce an error:
> https://docs.python.org/2/reference/simple_stmts.html#the-exec-statement
> 
> 
> Change-Id: I159cc1be58ff375f313ae5c4fb814763704b880e
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1893647
> Reviewed-by: Michael Achenbach <machenbach@chromium.org>
> Commit-Queue: Milad Farazmand <miladfar@ca.ibm.com>
> Cr-Commit-Position: refs/heads/master@{#64736}

TBR=machenbach@chromium.org,bmsdave@gmail.com,tmrts@chromium.org,miladfar@ca.ibm.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Change-Id: Ib62143645184d768f54272b7c2d7745f6b700369
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1921171
Reviewed-by: Milad Farazmand <miladfar@ca.ibm.com>
Commit-Queue: Milad Farazmand <miladfar@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#64995}

test/preparser/testcfg.py

@@ -68,7 +68,7 @@ class TestSuite(testsuite.TestSuite):
       return MkTest
     with open(pathname) as f:
       text = f.read()
-    exec text in {"Test": Test, "Template": Template}
+    exec(text, {"Test": Test, "Template": Template})
 
   def ListTests(self):
     result = []