small improvement for occasional Python user

[espinielli]

Being a dumb/occasional user of Python, I would like to suggest self contained examples like for example enhancing
the from_file() in traffic/core/mixins.py from

   Example usage:
   >>> t = Traffic.from_file("data/sample_opensky.pkl")

to

   Example usage:
   >>> from traffic.core import Traffic
   >>> t = Traffic.from_file("data/sample_opensky.pkl")

Similarly for other parts of the library implementing a from_file() method, like SO6 (from traffic.data) or Decoder (from traffic.data.decode)...

Suggesting this because I resolve to searching/browsing the source code every time... ;-)

[espinielli]

I also think that query() with f-string is not prominently documented enough.
Things like (think about looping over multiple airports):

from traffic.data import opensky
from traffic.data import airports

apt = airports["LTFM"]

ltfm = opensky.history(
   date,
   airport=apt.icao)
ltfm40_arr = (
    ltfm
    .distance(apt)
    .query(f"destination == '{apt.icao}' & distance <= 40")
    .eval()
    )