compat: don't rely on cpython-specific builtins manipulation

Pierre Augier signaled on the mailing list that this fails on Pypy and pointed
out the correct solution.

Differential Revision: https://phab.mercurial-scm.org/D11732

mercurial/pycompat.py

@@ -44,6 +44,7 b' if not ispy3:'
     FileNotFoundError = OSError
 
 else:
+    import builtins
     import concurrent.futures as futures
     import http.cookiejar as cookielib
     import http.client as httplib
@@ -55,7 +56,7 b' else:'
     def future_set_exception_info(f, exc_info):
         f.set_exception(exc_info[0])
 
-    FileNotFoundError = __builtins__['FileNotFoundError']
+    FileNotFoundError = builtins.FileNotFoundError
 
 
 def identity(a):