Pickletools is too restrictive on STOP opcode

[Legoclones]

Bug report

Bug description:

When the STOP opcode is encountered in pickletools, a check is made to see if there's anything left on the stack, and an error is thrown if there is.

cpython/Lib/pickletools.py

Lines 2542 to 2543 in 5414b97

	if stack:
	raise ValueError("stack not empty after STOP: %r" % stack)

However, both C and Python versions of the Unpickler don't make these checks and don't care how many items are left on the stack afterwards, as long as there's something for them to return.

In addition, C and Python versions of pickle don't care if a MARK opcode is still present or it's inside a metastack or whatnot, but since pickletools treats MARK as an object on the stack, it will also error out if a pickled bytestream like b'(K\x01.' is presented:

pickletools:  
    0: (    MARK
    1: K        BININT1    1
    3: .        STOP
highest protocol among opcodes = 1
pickletools failed stack not empty after STOP: [mark]

CPython versions tested on:

3.11

Operating systems tested on:

Linux

Linked PRs

• gh-127079: Remove check for items on stack #127080