Add a new route to the API

[alfredodeza]

The current API is not exposing country/{country} which needs to be implemented to list cities. The route should allow only GET HTTP requests with a JSON response providing information from the historical high and low for that country, city, and given month.

As with any implementation, this addition should include at least one test function to work with the pytest runner and test framework. The test should verify a 200 response from a correct request.