Allow calendar mode string to be flexible

isodatetime update: d4dd6d95adc9a46f892067f2c99e0e510bb9d9d1.
Remove hard coded calendar type string

Use constants in `isodatetime.data.Calendar` instead.
Fix import statement
Merge pull request #1022 from matthewrmshin/flexible-calendar-mode

Allow calendar mode string to be flexible