Fix UnicodeDecodeError on Windows by specifying utf-8 encoding

[ace-xc]

Problem
On Windows systems (where the default encoding is often GBK or CP1252), installing this package fails with a UnicodeDecodeError because setup.py attempts to read README.rst and VERSION without specifying an encoding. Python then uses the system default encoding, which cannot decode the UTF-8 characters in those files.

Error Message:

UnicodeDecodeError: 'gbk' codec can't decode byte 0xa5 in position 623: illegal multibyte sequence

Solution
I explicitly added encoding='utf-8' to the open() calls in setup.py. This ensures that the files are correctly read as UTF-8 regardless of the operating system's default encoding settings.

Verification
Verified that pip install . works correctly on Windows after this change.