Refactoring DB Manager Plugin to use list comprehensions and generators

[dericke]

Description

Within the DB Manager plugin, this PR replaces for loops with equivalent list comprehensions where possible, and some list comprehensions with generators. In two cases, I also squashed some conditionals together, and in one case I squashed dict population into the intial creation. This should make the altered methods somewhat faster.

[elpaso]

unrelated test failure

[m-kuhn]

Suggested change

	return u'.'.join(
	return '.'.join(

While reworking this, the u prefix is no longer needed.

[roya0045]

If this aims to modernize things, shouldn't we use the format or even f-string instead of the old % method for strings?

[dericke]

@roya0045 and @m-kuhn , I limited the types of changes I made to avoid making a huge PR, but I would be happy to do a more extensive rework of these files if that would be welcomed.

[roya0045]

My nitpick was mostly for one line. Though I'm not sure if this refactoring gives any advantage or is mostly a matter of style change. But I highly dislike the % formatting. But I don't think its worth changing them for the sake of it, at the moment, maybe except on the lines you modified.

[m-kuhn]

@roya0045 and @m-kuhn , I limited the types of changes I made to avoid making a huge PR, but I would be happy to do a more extensive rework of these files if that would be welcomed.

Sounds good to me 👍