bp2build: add python_binary_host converter.

This CL adds a simple python_binary_host to native py_binary converter
for standalone modules that don't depend on libs. It also adds support
for the conditional py2/py3 build target based on the version prop.

Test: milestone-2/demo.sh full
Test: bazel query 'kind("py_binary rule", //bionic/...)'
Test: bazel run //bionic/libc/tools:genfunctosyscallnrs
Test: bazel run //bionic/libc/tools:genseccomp
Test: go tests
Fixes: 182236395
Change-Id: Ibe5ec6cd0dc12a61b3a449a8c723d80b891fae42
diff --git a/bp2build/Android.bp b/bp2build/Android.bp
index 8deb5a2..99d706c 100644
--- a/bp2build/Android.bp
+++ b/bp2build/Android.bp
@@ -19,6 +19,7 @@
         "soong-bazel",
         "soong-cc",
         "soong-genrule",
+        "soong-python",
         "soong-sh",
     ],
     testSrcs: [
@@ -27,6 +28,7 @@
         "cc_library_headers_conversion_test.go",
         "cc_object_conversion_test.go",
         "conversion_test.go",
+        "python_binary_conversion_test.go",
         "sh_conversion_test.go",
         "testing.go",
     ],