Use str() instead of to_string(), as to_string() sometimes returns bytes

[robin-norwood]

pyethapp account list was giving the following error:

/pyethapp/app.py", line 586, in list_accounts
    click.echo(fmt.format(i='#' + to_string(i + 1),
TypeError: must be str, not bytes

This is because, on python 3, ethereum.utils.to_string() will, when given an integer, return bytes (!).

ethereum.utils.to_string should probably also be fixed, or at least given a more correct name, but that's more work than I'm willing to take on right now.

[robin-norwood]

FYI, I don't think the Travis failure is due to this change - it looks like setuptools has deprecated the use of sets in requires. I'll submit a separate pull request for that.