Respect users $PATH settings

[jottr]

Using #!/usr/bin/python3 assumes that users have python installed into /usr/bin/.
Many users use pyenv or something similar to set up their python interpreter.

This PR makes adb-sync more robust in these cases.

[googlebot]

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[jottr]

I signed it!

[jottr]

I signed it.

[googlebot]

CLAs look good, thanks!

[divVerent]

Looks good and helps with BSD compatibility. Thanks!