Broaden the type hint for `SQL.join()`

[rpmrmartin]

Passing a generator to SQL.join(), as suggested in the documentation for that method, produces errors in Mypy and Pyright:

import psycopg.sql as sql

sql.SQL(', ').join(sql.Identifier(n) for n in ['foo', 'bar', 'baz'])

Mypy:

Argument 1 to "join" of "SQL" has incompatible type "Generator[Identifier, None, None]"; expected "Sequence[Composable]"

Pyright:

Argument of type "Generator[Identifier, None, None]" cannot be assigned to parameter "seq" of type "Sequence[Composable]" in function "join"

Perhaps the seq parameter could be annotated with Iterable instead of Sequence.

[dvarrazzo]

I think you are right, yes. Will look into it (or, if you feel free, please send a MR, because you are right: an iterable is good)

[dvarrazzo]

Nevermind: done, together with another typing annotation issue. Thank you for the report.