I don't think a subreddit would be a good replacement for the forum. While Reddit provides the same basic functionality as a forum (you can make posts and reply), I feel like it's mainly a "content sharing" rather than a "discussion" site. On a forum, topics and posts are sorted purely by time; on Reddit the order is also influenced by up- and downvotes. This vote-based ordering works well for finding interesting content related to a topic, but isn't all that useful in a discussion environment (unless there are a lot of posts, which isn't the case for the Pythonista forum).
Reddit also makes it hard to see all recent comments in a subreddit - you can sort posts by new, but this shows the most recently posted posts rather than the most recently commented ones. The same goes for comments on a single post - because of the tree structure, sorting by new shows the most recent top-level comments, rather than comment chains with the most recent replies. Even when logged in, there's no way to see all unread posts/comments like on the forum.
On top of that, I'm not a huge fan of Reddit's web interface (the "new" one at least). Like most sites nowadays, clicking a link does some JavaScript magic instead of actually loading a new page, but it's not implemented very well. On desktop things are slow to load, and on mobile it doesn't work well on unreliable network connections (the tiniest network disconnect causes the entire page to be replaced with an "oopsie woopsie"-style error, and reloading will lose your current reading position). I know there are apps that probably work better than the website, but I really don't want to download an app for a site where I usually don't even log in. (I also like having multiple tabs.)
@omz I can totally understand if hosting the forum yourself takes too much time and work. In general I'm not against moving to a different website/service, the points above are about Reddit specifically. (Sadly I don't think there are any good alternatives though...) But tbh whereever the community decides to move, I'll probably go there too, unless it's Facebook or Google Groups.