Remove 'too many arguments' error when interpolating over mappings

[TheSignPainter98]

When no conversions are specified, interpolation over an empty tuple is accepted, but interpolation over an empty dict is rejected:

>>> 'foo' % ()
"foo"
>>> 'foo' % {}
Traceback (most recent call last):
  <stdin>:1:7: in <expr>
Error: too many arguments for format string

For context, extra dict arguments are not rejected when a conversion is present:

>>> '%(a)d' % {'a': 1, 'unused': 2}
"1"

This PR relaxes the 'too many arguments' check for mappings and adds tests to ensure consistency between tuples and dicts

[adonovan]

Thanks for this contribution! Code change looks sound.

Could you also add this line to the doc/spec.md file? Thanks.

After this paragraph:

If the "%" is immediately followed by "(key)", ...

add a new paragraph:

If the format string contains no conversions, the operand must be an empty tuple.

Thanks.

[TheSignPainter98]

Adding the line to the spec, I think the suggestion isn't quite right as an empty mapping would also be acceptable. Perhaps something like this would work?

If the format string contains no conversions, the operand must be a Mapping or an empty tuple.

[adonovan]

Thanks again!