tox.ini: Specify minimum version of click-man

Chandan Singhrequested to merge
chandan/min-version-click-man into master

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.

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