Explicit dependencies and avoid importing boto3

Hi,

Thanks for your work on boto3!

In this PR, I bring these two install-related items:

1. Read version information from `boto3/__init__.py` using a regex (thus without importing boto3 inside `setup.py`). This is how `pandas` does it. It makes the installation a bit neater. I had to adapt the regex to your way of expressing the version info in `__init__.py`.
2. add an explicit dependency on `bcdoc`.

As such, a simple `pip install boto3` just works. I tested it on a private pypi-server.

Hope it helps,
Regards,
Romain