Converting objects; e.g. support dataclasses?

[PallHaraldsson]

I think, though not sure, that dataclasses are very important in (modern) Python (supported in all current Python, i.e. since 3.7), and they basically map to Julia's structs (or well could), so do they need special support in this project? I don't see them in the table:

PythonCall.jl/docs/src/conversion-to-julia.md

Lines 25 to 40 in a61c022

	| `tuple` | `Tuple` |
	| `collections.abc.Mapping` (inc. `dict`) | `PyDict` |
	| `collections.abc.Sequence` (inc. `list`) | `PyList` |
	| `collections.abc.Set` (inc. `set`, `frozenset`) | `PySet` |
	| `io.IOBase` (includes open files) | `PyIO` |
	| `BaseException` | `PyException` |
	| `datetime.date`/`datetime.time`/`datetime.datetime` | `Date`/`Time`/`DateTime` |
	| `datetime.timedelta` | `Microsecond` (or `Millisecond` or `Second` on overflow) |
	| `numpy.intXX`/`numpy.uintXX`/`numpy.floatXX` | `IntXX`/`UIntXX`/`FloatXX` |
	| **Standard priority (other reasonable conversions).** | |
	| `None` | `Missing` |
	| `bytes` | `Vector{UInt8}`, `Vector{Int8}`, `String` |
	| `str` | `String`, `Symbol`, `Char`, `Vector{UInt8}`, `Vector{Int8}` |
	| `range` | `UnitRange` |
	| `collections.abc.Mapping` | `Dict` |
	| `collections.abc.Iterable` | `Vector`, `Set`, `Tuple`, `NamedTuple`, `Pair` |

I do see Python also has namedtuples, an alternative, not clear to me if either supported. I see tuples are, and something gets converted to Julia's NamedTuples. And PyDict. Python dictionaries are ordered, so I think PyDict supports that (converting from; and to Python likely also not problematic), otherwise mapping to Dict, simply would have been chosen?

[I see Python has now an internal C API (not public API) to get UTF-8 data, so maybe something better could be done for passing strings back end forth.]