Less-specific cutoff dates should default to the end of the specified time period instead of the beginning

[jonafato]

A use case I expect to be common here is using a date as the index name / datetime cutoff (the one documented in the readme), e.g. devpi index -c 20200101 .... Currently, devpi-timemachine parses this with datetime.fromisoformat, which returns the specified date at midnight. This means the index will return packages up to but not including the specified date; including packages published to PyPI on the specified date would be more intuitive. More generically, the plugin should determine how granular a the index name is as a datetime string and then adjust the cutoff to include up to the next day, hour, etc.