#35 Add a licensecheck-compatible license header to source files.

[Dave Tu]

We're using PySerial in the Chromium codebase, and we use the Debian licensecheck.pl script to semi-automatically detect licenses in many of our third party libraries. The files in this project already have license headers, but it would be helpful if they specified the type of license.

E.g. instead of:

# this is distributed under a free software license, see license.txt

Something like:

# this is distributed under the Python Software Foundation License version 2, see license.txt

Thanks!

[Chris Liechti]

the current sources at https://github.com/pyserial/pyserial include an SPDX header, is that sufficient?