Sync-ics2gcal

- overwrite events without updated field (80 81)
- drop python 3.6 support
- add python 3.10 support
- format with black (88)

migrate to poetry

* type annotations
* fire instead of argparse
* drop python 3.5
* add python 3.9

* default credentials support (docs [here](https://googleapis.dev/python/google-auth/latest/reference/google.auth.html#google.auth.default) and [here](https://developers.google.com/identity/protocols/application-default-credentials) )
* README updated

first version!