This should be grouped with the standard lib imports (just after 'import re')

It's probably worth bailing out early in the alternative case, to avoid doing a bunch of unnecessary work:

if self.get_latest_diffset() is None:
    return []

filediffs = ...

It might be worth using collections.defaultdict here, so you don't have to do the if rr_id in targets ... else ... thing.

You could probably get rid of the blank lines in this block and have it be more compact.

Can you wrap this conditional in () and get rid of the  characters?
I think it's probably also worth swapping around the 'not's:

if (rr_id != self.pk and
    entry['reviewrequest__reviews'] not in all_reviews):

Because you've got parentheses here, you don't need the \s

Can you use parens instead of \? Here and below.

Because this is inside [], you don't need the . Alignment is also a little bit weird here.

Remove this blank line.

No space between the key and :

Alignment is off here (indent the second line 1 more space).

Looking pretty good.
Did you install django-debug-toolbar? What database queries does this end up doing?

Spacing is a tiny bit off here--one more space before -, and one fewer after.

Wrap these in (), get rid of \, and fix alignment of the second line.

Get rid of this blank line.

Blank line between these.

Missing trailing period.

"most likely"
"knowledgeable"

"review request's"
We try to be explicit with that, since a "request" is often an HTTPRequest.

No blank line here.

You should call this only once and store it, since it's an otherwise expensive call.

You don't need a \\ when inside parens.
I'd suggest rewriting this as:
q = ReviewRequest.target_people.field.rel.through.objects
q = q.select_related('reviews')
q = q.filter(...)

The second row of parens should line up with the first.

Blank line between these.

Should be:

if (entry[...] not in all_reviews

Since we use entry['reviewrequest_reviews'] more than once, it'd be best to pull it out into a variable.

Should use != instead of not ==.

What's the contents of entry['reviewrequest_reviews']?

You already have this pulled out into a variable.

Comments should have sentence capitalization. Same with the other comments.

Can you add a comment block describing in detail the algorithm you're using?

Alignment problem with now and -.
Also, too many spaces after -.

Blank line before this.

Instead of a \\, you should use parens.

Instead of hard-coding 5 here, let's have a constant in the class definition. Something like MAX_GUESSED_REVIEWERS.

Since both of these are unique IDs, it's actually faster to not do this.$(..), and instead do $(..).

Blank line on either side of this.

No leading or trailing blank lines in the function.
Do you need to redo the .suggest-user query, or can you use allPeople above?

Not sure "Person" needs to be capitalized here. Maybe "the person"

You can use chaining to simplify this:
this._$targetPeople
    .inlineEditor('startEdit')
    .inlineEditor('appendToFieldValue...')

I saw the review request that adds this function. Is that really needed, or can you just implement that logic here? Ideally, it wouldn't need a new function.

Instead of using view in these callbacks, it'd be better to do:
cancel: _.bind(function() {
    // You can make use of 'this' now.     }, this)

No trailing blank line in the function.

Space between if and (.

No trailing blank line.

This looks pretty good! I have a couple small cleanup things to suggest, but otherwise, excellent!

Can you call this review_id, so it doesn't sound like a full-blown Review object?

No need for these outer parens now.

Kind of a big wall of text. Are there some good parts where you can break this up into separate paragraphs? Maybe after the first line, and at "If the user was targetted", and for the last line.

Alignment issue. The (weight / should be indented one more space.

This might be a bit nicer, and a little faster, by doing:
sorted_entries = sorted(
    weighted.items(),
    key=itemgetter(1),
    reverse=True)
sorted_users =[
    user
    for user, weight in sorted_entries[:self.MAX_SUGGESTED_REVIEWERS]
]

Should be consistent with the quotes and use single quotes for the delimiter.

Constants should be all capital letters, so something like RECENCY_WEIGHT and TARGET_WEIGHT.

This can be simply "if entry['user']:"