This commit provides support for virtualenv-like pyenv.cfg configuration files, where the `prompt` value is a plain text rather than a quoted string.
Before the commit, `prompt = My custom prompt` would not match the regex, returning a fallback of the $VIRTUAL_ENV directory name.
After the commit, both venv-like `prompt = 'My custom prompt'` and virtualenv-like `prompt = My custom prompt` are supported.
Closes#2628
