Change to calver scheme

[blaylockbk]

Xarray (and others) moved to the calendar versioning scheme (see pydata/xarray#6176). I do like the versioning with a date more than the major.minor.mirco versioning.

I would like to follow the YYYY.MM.DD scheme.

Instead of Herbie 0.0.11, the next release would be something like Herbie 2022.06.15

[blaylockbk]

This will be done in my next release...

Use the 4-digit year, digit month (doesn't need to be zero padded, because zeros are ignored in setuptool), and micro (for minor fixes made within the month)
YYYY.MM.micro

The first release using this scheme will be 2022.9.0