dev-libs/protobuf: enable python-3.6

dev-libs/protobuf: enable python-3.6

The package build with python 3.6

 * Package:    dev-libs/protobuf-3.3.0
 * Repository: vivovl
 * Maintainer: vapier@gentoo.org tomboy64@sina.cn,proxy-maint@gentoo.org
 * USE:        abi_x86_32 abi_x86_64 amd64 elibc_glibc java kernel_linux python_targets_python3_6 test userland_GNU zlib
 * FEATURES:   compressdebug preserve-libs sandbox splitdebug test userpriv usersandbox

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Testsuite summary for Protocol Buffers 3.3.0
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