setup.py: Get version from version.py

This creates a new file, [`pytest_httpbin/version.py`](https://github.com/msabramo/pytest-httpbin/blob/versionpy/pytest_httpbin/version.py) and makes `setup.py` execute that file rather than doing an `import pytest_httpbin` which causes the `setup.py` to depend on `pytest` being installed.

Fixes: #9

Cc: @kevin1024, @sylvinus