Breaking interface change wrongly released with 2.5

[csparpa]

I wrongly released a breaking change with 2.5, apologies for that!

Function webapi25.owm25.get_version used to return a string, I changed it so that it returned a tuple
Eg:

owm.get_version()  # used to return: "2.5.0" but now returns (2, 5, 0)

Returning a string should be restored and a deprecation should be introduced for that function and webapi25.owm25.get_API_version as well explaining that the return type will be modified on version 3.0.0 onwards

[csparpa]

Done with #161