Use exec() function in both Python 2 and Python 3 (#36)
Use exec() function in both Python 2 and Python 3 (#36)

Summary:
Legacy __exec__ statements are syntax errors in Python 3 but __exec()__ function works as expected in both Python 2 and Python 3.

This PR attempts to solve a corner case described in https://github.com/facebook/bistro/pull/34#issuecomment-513992204 where having a dict comprehension inside an __exec()__ function call can be problematic.

snarkmaster
Pull Request resolved: https://github.com/facebook/bistro/pull/36

Test Plan:
- Ran the new code:
```
$ jf get D16454731
$ ./cmake/targets_to_cmake_lists.py .
```
- Compared the `CMakeLists.txt` between the old & new version of the code.

Reviewed By: agoder

Differential Revision: D16454731

Pulled By: snarkmaster

fbshipit-source-id: bbdd6f330baf25716806f3750d870e104b393a39

facebook/bistro,facebook/bistro,facebook/bistro,facebook/bistro,facebook/bistro,facebook/bistro