audit: temporarily allow python3.9 rc

[dtrodrigues]

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Temporarily allow the python 3.9 release candidate In support of Homebrew/homebrew-core#61163 so that the audit doesn't fail.

[dtrodrigues]

Either there needs to be another allowlist for formulae that are allowed unstable but also don't yet exist, or the audit_spec test needs to be modified to ignore python 3.9 when checking to see if all of the formulae on the unstable allowlist exist, or maybe something else.
rspec ./test/dev-cmd/audit_spec.rb[3:43] # Homebrew::FormulaAuditor [email protected] formula exists

[dtrodrigues]

See: https://github.com/Homebrew/brew/blob/master/Library/Homebrew/test/dev-cmd/audit_spec.rb#L873

[dtrodrigues]

I ended up making a new list for this case since it somehow needed to be separated from the other unstable formulae in Homebrew since it won't exist in Homebrew core for some time.

[MikeMcQuaid]

Thanks @dtrodrigues!