Convert check_radio_versions to python3

Additionaly,
1. Run it as a python_binary_host so that it uses the hermetic python
toolchain
2. Fix an existing bug where if `board_info_txt` was empty, $^ would
   evaluate to its own path, and it would try to parse the relevant
   configuration from the wrong file

Bug: 196300985
Test: m out/target/product/vsoc_arm64/android-info.txt
Change-Id: Ie1e51a93041591829da4351dd57f37106e15d764