Issue #15166: Re-implement imp.get_tag() using sys.implementation.

Also eliminates some C code in Python/import.c as well.

Patch by Eric Snow with verification by comparing against another
patch from Jeff Knupp.

Lib/imp.py

@@ -11,7 +11,7 @@ from _imp import (lock_held, acquire_lock, release_lock,
                   init_builtin, init_frozen, is_builtin, is_frozen,
                   _fix_co_filename, extension_suffixes)
 # Could move out of _imp, but not worth the code
-from _imp import get_magic, get_tag
+from _imp import get_magic
 
 from importlib._bootstrap import new_module
 from importlib._bootstrap import cache_from_source
@@ -37,6 +37,11 @@ PY_CODERESOURCE = 8
 IMP_HOOK = 9
 
 
+def get_tag():
+    """Return the magic tag for .pyc or .pyo files."""
+    return sys.implementation.cache_tag
+
+
 def get_suffixes():
     warnings.warn('imp.get_suffixes() is deprecated; use the constants '
                   'defined on importlib.machinery instead',