Declare package version in 'setup.py' to enable installing 'requests' as a dependency

Importing the `braintree.version` module in `setup.py` will import the `braintree` package which in turn, by importing all its modules into its namespace will end in the chain of imports attempt to import `requests`. This prevents `setup.py` from working correctly if `requests` hasn't been previously installed manually.

The proposed change, by moving the package version declaration to `setup.py` and then reading that in `braintree.version` avoids this problem and enables the automatic install of `requests` as a dependency.