Provide the MIME name for encodings

Feature or enhancement

In many cases the name of the encoding matches the preferred MIME name, ignoring case and punctuation. But in some cases it is different (e.g. "ascii" vs "US-ASCII"). Python often accepts non-MIME names as well as MIME names, but other software can be more picky. Providing the preferred MIME name will allow to generate more standard documents (XML, emails, etc). We can add a public optional attribute mime_name to CodecInfo.

cc @malemburg

首先在 CPython 原始碼中定位 CodecInfo 和與編碼相關的測試，然後檢查編碼名稱如何表示與解析。當公開的選用 mime_name 屬性能夠提供偏好的 MIME 名稱（包括 ASCII 與 US-ASCII 這類情況），並且測試涵蓋這些不同名稱時，工作就完成了。