CLN Dont upcast where unnecessary (PDEP6 precursor)
precursor to https://github.com/pandas-dev/pandas/pull/50626/files, trying to reduce the diff

there's some example / tests in which upcasting from int to float happens, but where it's not the purpose of what's being shown / tested. similar to https://github.com/pandas-dev/pandas/pull/50493
thx @MarcoGorelli 