field named async generates invalid code

[brunnels]

Here's what gets generated:

    async: Optional[str] = field(
        default=None,
        metadata={
            "type": "Element",
            "nillable": True,
        }
    )

Here's what I edited it to fix it but I haven't tested that the resulting xml looks right yet.

    async_: Optional[str] = field(
        default=None,
        metadata={
            "type": "Element",
            "nillable": True,
            "name": "async"
        }
    )

[tefra]

closed in #602

Thanks for reporting @brunnels

[tefra]

I though I had pushed this one, sorry about, really closed in #607