support datetime[us]

[nwoolmer]

Repro:

https://github.com/questdb/py-questdb-client/tree/nw_datetime_us

from questdb.ingress import Sender, TimestampNanos
import pandas as pd


df = pd.DataFrame({'x': [1,2,3]})
df['timestamp'] = pd.Timestamp.now()

with Sender.from_conf("http::addr=localhost:9000;") as sender:
    sender.dataframe(df, table_name='abcd', at='timestamp')

questdb.ingress.IngressError: Unsupported dtype datetime64[us] for column 'timestamp'. Raise an issue if you think it should be supported