Nopo

Fix: El.is_selected

- Fix bug when converting to XPath when using CSS selector in El and Els
- Add Pg class for base page

**Full Changelog**: https://github.com/DingJunyao/nopo/compare/v1.2.0...v1.3.0

- modify wait methods from checking first selector to checking XPath of `El`
- add support for `By.LINK_TEXT` and `By.PARTIAL_LINK_TEXT` in `single_selector_to_xpath` method

- Add `ElParent` class above `El` and `Els`
- Try to add support for Python 3.7 and above

First stable version after practicing and fixing for serval months. Enjoy it!

- Modify `python_requires` to `>=3.9`
- Fix type of `by` in `__init__` of `El` and `Els`
- Import `By` of Selenium in `nopo.__init__` so that you can use `from nopo import By` to import `By`.

Fix requirements.

Sorry for not including them in previous versions. I never found the mistake until two days ago, but I didn't correct it until now.