Add missing boto3 dependency

[jak574]

functions-python seems to assume that boto3 is installed already, and does not have it as a dependency in either the pyproject.toml or Pipfile file.

As boto3 is a dependency, this pull request adds it.

[CLAassistant]

All committers have signed the CLA.

[lpsinger]

@ryanblock, was the boto3 dependency omitted on purpose because it is preinstalled on the AWS Lambda runtime images?

[ryanblock]

@lpsinger yes, thanks for pointing that out. @jak574, we omit Boto3 as a dependency for the above reason; you don't want to install it as a dependency during deployment, as it will take up an enormous amount of your code payload, slow down cold starts, etc. It is assumed that you already have Boto3 installed, but if current docs are insufficient, let's open a PR there!