Drop Python 3.5 and 3.6 support

[tswast]

Pandas has already dropped 3.5 support: pandas-dev/pandas#29034

This would allow us to use f-strings among other features.

[tswast]

For reference: https://numpy.org/neps/nep-0029-deprecation_policy.html#support-table

3.6 support was also dropped. pandas-dev/pandas#35214

[max-sixty]

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