Fix pystruct incompatibility and Add test_buffer.py from cpython 3.8 - #3196

This revision fix the incompatibility behaviour in pystruct and add test_buffer.py from cpython 3.8.

There exists incompatibility when overflow occurred in struct.pack. Here is example

cpython

>>> import struct
>>> struct.pack('b', (1<<128)-1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
struct.error: argument out of range

rustpython

>>>>> import struct
>>>>> struct.pack('b', (1<<128)-1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OverflowError: Python int too large to convert to Rust i8

This minor difference make test_buffer.py stalled in this lines.

try:
    struct.pack(fmt, (1<<exp)-1)
    break
except struct.error:
    pass

test_buffer.py is use struct and _testbuffer module which is not yet implemented. Thus this test will be skipped in default

In addition, test_add_file_after_2107 in test_zipfile.py and test_b85decode_errors & test_a85decode_errors in test_base64.py now going works