BAMS paper version in py3

Same package as the one used for the paper "Evaluating climate models with the CLIVAR 2020 ENSO metrics package" published in BAMS (https://doi.org/10.1175/BAMS-D-19-0337.1) but now available in python3.

There are no significant differences in the metric values between the python2 and python3 versions.

The released code is the same as in v1.1-2021 but only with more standard way of software versioning rule, following semantic versioning..