Python3 fixes

[psteinb]

I installed the module through pip3 - which didn't throw an error while doing so. Only after the fact, I found out that this module is python2 only. I started to port it to python3. Please do not merge yet, as I haven't concluded my tests.

[psteinb]

added a small flag that allows to filter per row (not yet ported to geojson), this PR is now ready for review and potential merge

[amotl]

Dear Peter,

we just wanted to let you know that the renovated version of this fine program now officially started living at [1]. We would love to hear about further feedback from you.

If you still have interest in this program, we would welcome any contributions over there and will especially be happy to discuss making this ready for Python3 and adding more filtering capabilities ¹.

With kind regards,
Andreas.

[1] https://github.com/hiveeyes/dwdweather2

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¹ Re. "more filtering capabilities". While doing it in the code will be an option, we are also thinking about improving the JSON output capabilities in order to support improved filtering through jq which might be more flexible in the end. However, this might depend on the respective use case scenario. Just let us know if this might spark your interest.