v0.2.1

Quick release to bump matrix-nio to version >=0.18, as otherwise that is required for the bot to work with Synapse 1.38+.
See #86 for details on the bug.

Otherwise, full changelog below:

• Improve the '!listreminders' output.

• Add one-letter aliases to each command.

• Add 'remind' as an alias for the 'remindme' command.

• Use a pill when reminding a user.

• Timezone errors due to daylight savings times will be corrected after a bridge restart.

• Prevent timezone-related errors when creating a reminder.

• Better parsing of reminders that have newlines.

• Add release instructions.

• Update setup.py to indicate Python 3.6+ is required.

• Bump minimum version of matrix-nio to 0.18.

• Bump the version of libolm to 3.2.4.

• Bump the version of Python in the CI to 3.9.