dropped compatibility with python<3.7; added pyproject.toml to specif…

…y build dependencies & build backend; fixed export when output and temporary directory are on different filesystems

sha256 sums:
b7fc98285ca02e4b9495dfeb9286e68d6e3ecd9eb717682f7913950fa86aad7a ical2vdir-1.0.0-py3-none-any.whl
1234f9e5da39f5c875fec97b13dbf08487a98af0a73bebb042364ee7910d1718 ical2vdir-1.0.0.tar.gz

restore compatibility with python3.5

fixed:

- only write to disk when creating or updating item
- cleanup temporary files

first prototype