Bump version: 0.12.1-dev.4 → 0.12.1

Prepare v0.12.0, conditional bump version (#352)

## Overview

Changes:

* Updated the release version string (v0.12.0) and date (today)
* Updated versioning to use SemVer v2.0.0
* Update bump-version action to bump when changes are made to specific
files on the master branch

## Related Issue / Discussion

The version scheme and bumping setup is the same as can be found for
xclim and other Ouranos projects.

## Additional Information

https://semver.org/spec/v2.0.0.html

Bump version: 0.11.3 → 0.11.4

Bump version: 0.11.2 → 0.11.3

Bump version: 0.11.1 → 0.11.2

Bump version: 0.11.0 → 0.11.1

Bump version: 0.10.0 → 0.11.0

Bump version: 0.9.2 → 0.10.0

Bump version: 0.9.1 → 0.9.2

Bump version: 0.9.0 → 0.9.1