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 源代码中定位 CodecInfo 和与编码相关的测试，然后检查编码名称是如何表示和解析的。当公开的可选 mime_name 属性能够提供首选 MIME 名称（包括 ASCII 与 US-ASCII 这类情况），并且测试覆盖这些不同名称时，工作就完成了。