Now that MinGW and MSYS have been removed from the Windows builders,
bash is not available to run the previous upload_nightlies.sh script.
Convert the script to Python 2 for cross-platform support.
Additionally, switch to the `boto3` library for easy uploading
without needing to install `s3cmd`,
and move the code into mach for easy `boto3` installation
as the new `./mach upload-nightly` command.

Also, hard-code the paths to the packages instead of using
globs to look for them, as the paths are static.
(The paths used to contain timestamps,
but we now insert timestamps when uploading to S3
to improve reproducibility.)