a better way to install opensky package for python

[mouhsineelachbi]

The correct way to install opensky packages using pip and according to #634885026
pip install -e "git+https://github.com/openskynetwork/opensky-api.git#egg=python&subdirectory=python"

[jonas-eckhardt]

You can also install it via the following command. This does not require ssh.

pip install https://github.com/openskynetwork/opensky-api/archive/master.zip#subdirectory=python

[QwertyZero99]

Thx

[jaluebbe]

Added a similar install instruction to the README.