mirrorlist: Optimize the setup_continents method.

Profiling before & after:
```
ncalls  tottime  percall  cumtime  percall filename:lineno(function)
     1    0.000    0.000    0.005    0.005 mirrorlist_server.py:290(setup_continents)
     1    0.001    0.001    0.001    0.001 mirrorlist_server.py:291(setup_continents)
```

:earth_americas: :runner: :speedboat: :racehorse: :dash: :earth_asia: