Remove 32bits support

[myselfhimself]

Reasons for dropping the 32bits support:

• Non-debugged 32-bits numpy/PIL-related test suite error only for Manylinux 32 bits Python >=3.6 https://github.com/myselfhimself/gmic-py/runs/998946919?check_suite_focus=true
• No desire to spend to spend time debugging on a 32bits Linux VM
• If a targetted related software such as Blender had been supporting 32bits we might have continued doing so as well, obviously Blender >2.80 dropped 32bits support, per its release archive and this related tracker issue.
• We can still help the community in building 32bits versions, it is just that it will be unsure if the pytest test suite will pass 100% on it