Use python3 in various python scripts in libcore/

Note: scripts in tools/testmapping/ are not migrated to python3 yet,
because apparently it's broken even if I run on python2 and follow the
instructions in the README.

Bug: 185116539
Test: libcore/benchmarks/src/benchmarks/ImtConflictBenchmarkGen.py 10
Test: `source libcore/tools/docs/crypto/run_update_crypto_support.sh 31` succeeds if not running activity mode
Test: libcore/tools/docs/crypto/update_crypto_support_test.py
Test: libcore/tools/docs/crypto/format_supported_algorithm_table.py libcore/tools/docs/crypto/data/crypto_support.json
Change-Id: Icab833568c76cb20b1ff9ad9ddba3f9f459b0641
4 files changed