Test coverage c extension doc fix

Currently in the test coverage section, specifically 5.2.1.4, it mentions building the C extension for coverage.py. For a while now coverage.py has come with the C extension by default, so this step is almost never required (see here: https://coverage.readthedocs.io/en/coverage-5.1/install.html#c-extension).
I'm thinking it would be worth mentioning in the guide that installing the C extension is only required if 'coverage --version' says 'Without C extension'.

从测试覆盖率部分开始，具体查看小节 5.2.1.4，并将其中关于 C 扩展的说明与链接的 coverage.py 文档进行比较。当指南说明只有在 coverage --version 报告 “Without C extension” 时才需要安装该扩展，即表示完成。