Change read_tmy3 parameter input order

[AdamRJensen]

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Based on this #1779 (comment), it seems there is consensus that the map_variable parameter should be the last or second to last input parameters. This PR updates this for the read_tmy3 function (variable mapping of read_tmy3 has yet to be officially released, hence this PR is not a breaking change).

[kandersolar]

I think it might make sense for technical mumbo-jumbo parameters like url and encoding to always be at the end of the parameter list. They aren't really "features" per se, they're just annoying "make it work" buttons that one resorts to using when something external to pvlib forces your hand.