Freeze a README link to rstlint.py (#6)

README.md links to rstlint.py in a dynamic head of `main` CPython
branch:

```
    https://github.com/python/cpython/blob/main/Doc/tools/rstlint.py
                                           ^^^^
```

However, after python/cpython#31097 is merged,
the link will stop working. To keep the link working, `main` needs to be
replaced with some concrete commit SHA.

Author: arhadthedev

README.md

@@ -1,7 +1,7 @@
 # Sphinx Lint
 
 Sphinx Lint is based on [rstlint.py from
-cpython](https://github.com/python/cpython/blob/main/Doc/tools/rstlint.py).
+cpython](https://github.com/python/cpython/blob/e0433c1e7/Doc/tools/rstlint.py).
 
 
 ## What sphinx-lint is, what it is not?