Patch version 2 to take Serhiy's review in account:

* complete Objects/lnotab_notes.txt update. Replace (300, 300) delta example with (300, 200) delta for better readability (300 is the bytecode offset delta, 200 is the line number delta)
* fix added assertion in peephole: don't check the reference counter for empty byte string (which can be the empty byte string singleton)
* restore addrmap name in peephole
* don't change importlib MAGIC: we only change it between Python minor versions, it was already changed for Python 3.6a0
* assemble_lnotab() divide with positive numbers to avoid undefined behaviour on C
* dis.py: use "if line_incr >= 0x80: line_incr -= 0x100" instead of struct.unpack() to convert unsigned to signed

Note: avoid also useless "if (x != NULL)" checks before calling PyMem_Free(). PyMem_Free(NULL) is well specified: do nothing.