TYPE_INT64 is supported in the marshal module since Python 1.5. Since its use causes instability of marhalled files produced on different platforms, the marshal module in Python 3.3 no longer use it when write new files (see issue15466). And in Python 3.4 it stopped reading files containing the TYPE_INT64 code (see issue15480).

This is backward incompatible change. Python 2.7 still produce files containing the TYPE_INT64 code (though this code is rare, because integers in ranges -9223372036854775808..-2147483649 and 2147483648..9223372036854775807 are rare), and there may be marshal files produced by older versions of Python 3, Python 2 and Python 1. Formally Python 3 supports marshal formats produced by all previous versions, except the code objects and the TYPE_INT64 code.

Supporting loading the TYPE_INT64 code don't create problems like using it for saving data. But it is needed for backward compatibility.

I think that removing the TYPE_INT64 code was a mistake.

Example:

$ python2 -c 'import sys, marshal; marshal.dump(1234567890, sys.stdout)' | python3 -c 'import sys, marshal; print(marshal.load(sys.stdin.buffer))'
1234567890

$ python2 -c 'import sys, marshal; marshal.dump(123456789012345, sys.stdout)' | python3 -c 'import sys, marshal; print(marshal.load(sys.stdin.buffer))'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ValueError: bad marshal data (unknown type code)

 python2 -c 'import sys, marshal; marshal.dump(12345678901234567890, sys.stdout)' | python3 -c 'import sys, marshal; print(marshal.load(sys.stdin.buffer))'
12345678901234567890

PR 4381 reverts this change and simplifies the restored code.

New changeset 00987f6230fcdbecc8d9ab4b2b9fae8f99a1a4a9 by Serhiy Storchaka in branch 'master':
bpo-32011: Revert "Issue #15480: Remove the deprecated and unused TYPE_INT64 code from marshal." (#4381)
https://github.com/python/cpython/commit/00987f6230fcdbecc8d9ab4b2b9fae8f99a1a4a9

New changeset d15bb5fcad584e113836486d17c6abcbf2168a86 by Serhiy Storchaka (Miss Islington (bot)) in branch '3.6':
bpo-32011: Revert "Issue GH-15480: Remove the deprecated and unused TYPE_INT64 code from marshal." (GH-4381) (#4405)
https://github.com/python/cpython/commit/d15bb5fcad584e113836486d17c6abcbf2168a86