Allow inserttable() to take columns as parameter

[Cito]

Suggested on the mailing list 2017-01:

The method Connection.inserttable(table, values) in the classic module might take more parameters, e.g. columns, similar to the Cursor.copy_from(...) method in the DB API 2 module.

(copied from Trac ticket 73, created 2018-04-23)

[Cito]

Actually I don't think a columns parameter would be much useful here and only tarnish the pleasant simplicity of this method. You can easily remove superfluous columns from the values argument in Python before passing it to the method. And the other copy_from parameters are not applicable here anyway.

(copied from Trac 2019-04-25)

[Cito]

Justin: I don't think we're talking about the same thing. The request was to allow inserting columns "a,c" into a table with columns "a,b,c,d" (we need to do this all the time). That's needed in case columns b,d have default values (otherwise NULL would work fine).

Doesn't this just need to accept an optional list of column names and join them with commas?
sprintf(buffer, "copy %s from stdin", table);
to allow:
COPY t(i) FROM stdin

I don't know for sure if we'd be able to use this for either of our loaders, but I suspect it would be competitive or better than prepared statements, for use cases where it can be applied.

(copied from Trac 2019-05-14)

[Cito]

See discussion on the mailing list 2020-06-20, will be released in 5.2.