tox.ini: Specify minimum version of click-man

`click-man` versions < 0.3.0 do not properly support multiple
entrypoints. Since this was added to `tox` after `0.3.0` was released,
`tox` should never be pulling older versions. But, let's add it here for
documentation purposes.

See
!1107 (comment 135187046)
for some background on this.