[lldb/Plugins] Move member template specialization out of class

This patch should fix the build failure that surfaced when build llvm
with GCC: https://lab.llvm.org/staging/#/builders/16/builds/10450



GCC complained that I explicitely specialized
 `ScriptedPythonInterface::ExtractValueFromPythonObject` in a
in non-namespace scope, which is tolerated by Clang.

To solve this issue, the specialization were declared out of the class
and implemented in the source file.

Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>