This has also been reported as a bug in the font-package, at bug 694493.
I tried to make a search for Type1, and found a definition from adobe, which referred to a definition of "number" for postscript, and a postscript reference (from adobe) told me that the character used should be a ".", so from my brief look, this appears to me to be a bug in the font-package.
Of course, one might argue that the matplotlib library should handle the error more gracefully, but the actual error might be in the afm file.
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