Dependency pexpect should be updated to avoid conflict with async keyword

[Svetlitski]

pexpect should be updated to at least version 4.7, as prior versions used async as the name of a function argument, preventing them from running on newer versions of python, which have async as a reserved word. If this is not possible, then the README should at least mention that you'll need to install an old (<=3.6) version of python in order to successfully use this repo.

[mutux]

Thanks! I ran into the same problem and fixed it by upgrading pexpect to version 4.7 (python==3.7)