Unfortunately you can’t bold symbols using the bold font, see this question on tex.stackexchange.
As the answer suggests, you could use \boldsymbol to bold phi:
r'$\boldsymbol{\phi}$'
You’ll need to load amsmath into the TeX preamble:
matplotlib.rc('text', usetex=True)
matplotlib.rcParams['text.latex.preamble']=[r"\usepackage{amsmath}"]