Incorrect encoding when reading UTF-8 markdown files

[sheyifan]

Bug: Incorrect encoding when reading UTF-8 markdown files on Windows

Description

When running md2cf on Windows, markdown files encoded in UTF-8 are read with the system default encoding (typically GBK/CP936 on Chinese Windows) instead of UTF-8. This causes Unicode characters (especially non-ASCII characters like Chinese, Japanese, etc.) to be incorrectly decoded and displayed as mojibake on Confluence pages.

Environment

• OS: Windows (Chinese locale, default encoding: GBK/CP936)
• Python version: 3.x
• md2cf version: 2.3.0

Steps to Reproduce

1. Create a UTF-8 encoded markdown file with non-ASCII characters (e.g., Chinese):

   # 需求

2. Run md2cf to upload this file to Confluence:

   python -m md2cf --host <confluence-url> --username <user> --token <token> --space <space> file.md

3. Check the page title on Confluence
   Expected Behavior
   The page title should display correctly as "需求" (Requirements in Chinese).

   Actual Behavior
   The page title displays as mojibake: "需求"

   This occurs because Python's open() function on Windows uses the system default encoding (GBK/CP936) when no encoding is specified, causing UTF-8 bytes to be incorrectly decoded.