Drop python2 support from Mesa

Ubuntu has dropped the python symlink to python2 (instead of redirecting it to python3 like other distros are doing), which means that if we want to build Mesa on Ubuntu we need the python3 shebang.

All of our scripts were already python3-compatible, with the exception of src/gallium/tools/trace/dump.py and bin/perf-annotate-jit.py, but neither of these are used by any build systems, so they can be updated later.

/cc @vlee who reported the issue in the first place
/cc @dbaker to review the python code
/cc @tpalli to confirm that the Android build system is able to use python3