Convert license tags to SPDX identifiers

This commit also converts our own copyright headers to directly use
SPDX, but leaves all other copyright header intact, only adding the
SPDX ident. As far as possible this commit also merges multiple
Data61 copyright statements/headers into one for consistency.

python2 --> python3

Update all scripts and build system to call python3, given python2's
upcoming doom. Use sys.maxsize instead of sys.maxint in one script
(maxint does not exist in python3).

style: run autopep8 on python files

manual: fix script licenses

manual: Build system compatible with python3

manual: gen_env: python3 compat

Use usr/bin/env for all python bangpaths to enable virtualenv use.

Very useful with python3 as the default platform.

Release snapshot