I am currently using Powerline on my Arch machine with Python 3.5 (just updated several weeks ago). The colors on the left of the tmux statusbar (using Solarized theme) is different from what it used to be in Python 3.4. Here is a screenshot:

Here is my machine's information:
OS: 4.2.5-1-ARCH
Tmux Version: tmux 2.1
Powerline Version: 2.2 & 2.3
Python Version: 3.5.0
Pip Version: 7.1.2
Hope there will be a fix soon.
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Have you tried TERM=xterm-256color tmux? Or tmux -2?
Setting TERM to either tmux-256color, xterm-256color or screen-256color should work in most cases.
Most helpful comment
Have you tried
TERM=xterm-256color tmux? Ortmux -2?