Ensure PyPi versions work

[bcollazo]

It seems Gym 0.25 now uses "render_modes" instead of "render.modes" which was yielding this error: #55 (comment).

Quick fix here is to add a CI check that ensures at least standalone catanatron-gym is installable and usable, and to lock the required gym to gym==0.21.0.

[coveralls]

Pull Request Test Coverage Report for Build 2687185135

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 95.54%

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Totals
Change from base Build 2687140997:	0.0%
Covered Lines:	1264
Relevant Lines:	1323

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💛 - Coveralls