Consider changing URLs to https in docs

Many URLs in the documentation are using the HTTP protocol:

```
cpython/Doc$ git grep "http://" | wc -l
460
cpython/Doc$ git grep "https://" | wc -l
805
```

A number of these could be converted to HTTPS. If this would be welcome, I am willing to open a PR for that, converting them with a script that checks if the HTTPS equivalent is reachable.