Namespace packages don't work on older Pythons

[jaraco]

Due to the way pip installs namespace packages, relying on .pth files to satisfy namespace packages, and because .pth files are only honored in site-packages, rwt cannot load packages that are namespace packages on older Pythons like 2.7.

$ python3.5 -m rwt backports.functools_lru_cache -- -c "import backports"
Loading requirements using backports.functools_lru_cache
$ python2.7 -m rwt backports.functools_lru_cache -- -c "import backports"
Loading requirements using backports.functools_lru_cache
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ImportError: No module named backports