I wasn't planning on any big features for Coverage.py 4.1, but I ended up making a big change. tl;dr: branch coverage is implemented differently, and as a result, your coverage totals are slightly different. Try it: Coverage.py 4.1 beta 1.
Because of Python 3.5's async and await keywords, the existing branch analysis based on bytecode was completely out of gas. The code had long felt jury-rigged, and there were long-standing bugs that seemed impossible to solve. The async features compiled very differently than their synchronous counterparts, and I didn't see a way to bring them into the bytecode fold.
So I ripped out the bytecode analysis and replaced it with AST (Abstract Syntax Tree) analysis. I like it much better: it's based on the structure of the code that people see and can reason about. Four old bugs were fixed as a result, along with the two or three new bugs reported on Python 3.5.
As a result though, coverage.py now calculates totals differently, because the full set of possible code paths is different. So your results will likely shift a little bit, especially if you are using branch measurement. They might be higher, they might be lower. For example, class docstrings now count as executable statements (because they are executable statements), and paths through "except" clauses probably were being overlooked.
Please try it out and let me know if you see anything wrong. I won't be surprised if there are complex code structures that are analyzed wrong somehow. Coverage.py 4.1b1: you know you want it...