Airports

- Add support for Ruby 3.0 (timrogers, with thanks to kleinjm)
- Add Misrata Airport (`MRA`) in Libya (timrogers, with thanks to a21-kevin)
- Drop support for Ruby 2.4 and 2.5, which are no longer officially supported (timrogers)
- Remove unnecessary dependencies from the gemspec, moving them to the `Gemfile` (timrogers)

- Add an entry for Berlin Brandenburg Airport (`BER`), which is missing from [OpenFlights](http://openflights.org) (PavelWeSki)
- Update airport data with the latest from [OpenFlights](http://openflights.org)

- Update airport data with the latest from [OpenFlights](http://openflights.org)
- Refactor how incorrect or missing data from OpenFlights is patched (MatjazKavcic)

- Support looking up airports by the name of the city they
are located in with `Airports.find_all_by_city_name` (viral810, timrogers)
- Refactor `Airports` so `Airport` objects are only generated once
and the code is cleaner (timrogers)

- Correct the time zone of Istanbul Airport (`IST`) (aliismayilov)
- Correct the name of Aviador Carlos Campos Airport (`CPC`) (jpgarritano)

- Drop support for Ruby 2.2 and 2.3, which are no longer officially supported (timrogers)
- Test against Ruby 2.6 (timrogers)
- Convert "nulls" in the source data, expressed as "\\N", to Ruby `nil`s (gabebw)