Fix running tests (#33719)

Fix running tests (#33719)

without that code I have stack trace on Python 3.7.1
when run tests\runtest.cmd /coreclr Top200
```
Traceback (most recent call last):
  File "runtest.py", line 2450, in <module>
    sys.exit(main(args))
  File "runtest.py", line 2423, in main
    lambda path: do_setup(host_os,
  File "runtest.py", line 550, in create_and_use_test_env
    ret_code = func(None)
  File "runtest.py", line 2433, in <lambda>
    test_filter_path))
  File "runtest.py", line 2356, in do_setup
    build_test_wrappers(host_os, arch, build_type, coreclr_repo_location, test_location, None, test_filter_path)
  File "runtest.py", line 1871, in build_test_wrappers
    delete_existing_wrappers(to_unicode(test_location))
  File "runtest.py", line 1822, in to_unicode
    return str(s, "utf-8")
TypeError: decoding str is not supported
```

* Simplify string conversion and update Python
* Fix path to Python3 executable

It was discovered during testing CoreRT in
https://github.com/dotnet/corert/pull/8021