Backport mpmath's fix for py.test 

from https://github.com/fredrik-johansson/mpmath/pull/17

Was:
```
$ py.test .
============================================================================= test session starts ==============================================================================
platform linux2 -- Python 2.7.3 -- pytest-2.2.4
collected 0 items / 1 errors 

==================================================================================== ERRORS ====================================================================================
______________________________________________________________________________ ERROR collecting . ______________________________________________________________________________
/usr/lib/python2.7/dist-packages/py/_path/common.py:315: in visit
>       for x in Visitor(fil, rec, ignore, bf, sort).gen(self):
/usr/lib/python2.7/dist-packages/py/_path/common.py:361: in gen
>               for p in self.gen(subdir):
/usr/lib/python2.7/dist-packages/py/_path/common.py:351: in gen
>                   if p.check(dir=1) and (rec is None or rec(p))])
/usr/lib/python2.7/dist-packages/_pytest/main.py:484: in _recurse
>       ihook.pytest_collect_directory(path=path, parent=self)
/usr/lib/python2.7/dist-packages/_pytest/main.py:140: in call_matching_hooks
>       plugins = self.config._getmatchingplugins(self.fspath)
/usr/lib/python2.7/dist-packages/_pytest/config.py:289: in _getmatchingplugins
>       plugins += self._conftest.getconftestmodules(fspath)
/usr/lib/python2.7/dist-packages/_pytest/config.py:188: in getconftestmodules
>                       clist.append(self.importconftest(conftestpath))
/usr/lib/python2.7/dist-packages/_pytest/config.py:217: in importconftest
>           self._conftestpath2mod[conftestpath] = mod = conftestpath.pyimport()
/usr/lib/python2.7/dist-packages/py/_path/local.py:532: in pyimport
>           __import__(modname)
sympy/mpmath/conftest.py:8: in <module>
>   rootdir = py.magic.autopath().dirpath()
/usr/lib/python2.7/dist-packages/py/_apipkg.py:112: in __makeattr
>           raise AttributeError(name)
E           AttributeError: magic
                                                                                DO *NOT* COMMIT!                                                                                
=========================================================================== 1 error in 1.01 seconds ============================================================================
```