bpo-45120: Updated windows 'cp' encodings to match 'bestfit' specification.

There is a mismatch in specification and behavior in some windows encodings.

Some older windows codepages specifications present "UNDEFINED" mapping, whereas in reality, they present another behavior which is updated in a section named "bestfit".

For example CP1252 has a corresponding bestfit1525:

• CP1252: https://www.unicode.org/Public/MAPPINGS/VENDORS/MICSFT/WINDOWS/CP1252.TXT

• bestfit1525: https://www.unicode.org/Public/MAPPINGS/VENDORS/MICSFT/WindowsBestFit/bestfit1252.txt

They have the following differences:

• In CP1252, bytes \x81 \x8d \x8f \x90 \x9d maps to "UNDEFINED".

• In bestfit1252, they map to \u0081 \u008d \u008f \u0090 \u009d respectively.

In the Windows API, the function 'MultiByteToWideChar' in "cp1252 mode" exhibits the bestfit1252 behavior.

This issue and PR proposes a correction for this behavior, updating the windows codepages where some code points where defined as "UNDEFINED" to the corresponding bestfit mapping.

For example: "b'\x81'.decode('cp1252')" produces:

>>> b'\x81'.decode('cp1252')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.8/encodings/cp1252.py", line 15, in decode
    return codecs.charmap_decode(input,errors,decoding_table)
UnicodeDecodeError: 'charmap' codec can't decode byte 0x81 in position 0: character maps to <undefined>

Where, i think, the intended behavior would be:

>>> b'\x81'.decode('cp1252')
'\x81'

This PR:

• updates the Makefile in Tools/unicode/ adding 'windows-bestfit'. Adding some pre-processing to the files bestfit mapping files downloaded from ftp.unicode.org.

• Adds the direct output from the gencodec.py corresponding to the cp encodings in Lib/encodings/

https://bugs.python.org/issue45120