CI: Enable Spellcheck in dockbuild
This is the initial PR to try and fix the issue #21354 the sphinx spelling extension needs every name to be divided and added to each line in order to get the name marked as spelt correctly. The file `names_wordlist.txt` is updated on every run of the function `get_authors()` located inside`scripts/announce.py`.

Since the issue #21396 is related to my previous PR I attempted to resolve this issue here as well, I followed the suggestion of @jorisvandenbossche and added a try/except to the `conf.py` if the dependency doesn't exist then nothing will happen.

@datapythonista since you submitted a PR(#21397) to try and fix the issue, perhaps you have an opinion about this bit of code. Also, if you would like to do the changes on your own PR I will just delete this bit and use your solution 😄 👍 

I would like some feedback on this first attempt, the generator `[names.extend(re.sub('\W+', ' ', x).split()) for x in cur.union(pre)]` seems a bit smelly but does the trick, should this be refactored to use a long form and make it more readable? I extended the list to names because split() was creating a list inside the main list and this makes every name to be split and added to the same list (this is probably very memory inefficient though)

I tried to run the script from my machine but I got an issue with it, probably I'm doing something wrong but when I run `./scripts/announce.py $GITHUB v0.15.1..v0.23.0` I get the error message:

```
usage: announce.py [-h] [--repo REPO] revision_range 
announce.py: error: unrecognized arguments: v0.15.1..v0.23.0
```

Not sure what I'm doing wrong, to be honest. If I run just the `get_authors()` function everything seems to be fine (including the v0.15.1..v0.23.0 format) and the names_wordlist is updated successfully.

I will look forward to read your opinions about this PR 👍 
----
- [ ] closes #21396 #21354
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry