Update docs with Bootstrap 4/Deform 3/Python 3 references

[stevepiercy]

When Deform 3.0 is approaching release, review all the docs and docstrings for any references to obsolete versions and update accordingly.

[slav0nic]

time to think about Bootstrap 5 integration :)

[stevepiercy]

Indeed, that is in the back of my mind. I would support developing on main the switch to BS 5. Many of the updates in Deform for Bootstrap 4 apply to Bootstrap 5, mainly the switch to flexbox. I don't know what else needs to be updated.

At the rate that I have spent on developing Deform, Bootstrap 6 will be released before we can release Deform 3. 😉