Convert dew point units to match temperature units

[lymanepp]

I noticed that the dew point units don't match the temperature:

"dewpoint": {
    "unitCode": "wmoUnit:degC",
    "value": 22.222222222222221
},

Ideally, the pynws API would use Celsius for all response values. But some of the NWS responses contain values in strings such as: Showers and thunderstorms likely. Mostly sunny. High near 98, with temperatures falling to around 95 in the afternoon. Heat index values as high as 106. Southwest wind 5 to 10 mph. Chance of precipitation is 60%. New rainfall amounts less than a tenth of an inch possible.

So making all degree units in a forecast response match the temperature seems like the least-bad approach.

[MatthewFlamm]

This LGTM. Thanks! This project doesn't do a very good job with integers vs. float, particularly when there are conversions. This PR is consistent with the current code though.

[MatthewFlamm]

Unless there is anything else you intend to add, I can make a release.

[lymanepp]

I don't have any other changes. Ready for a release that I can use to update the HA PR. Thanks!