I believe a recent PR made it such that your python version in asv.conf.json is no longer applied if there is no other python constraint. See context below:

@HaoZeke @mattip sorry if I'm missing something, but it looks like if you do not have an environment file, the python version specified in asv.conf.json no longer gets passed to mamba after this change. (ie if there is no python= constraint before this line runs, we never add one) Perhaps we should restore the python={self._python} in the initialization of mamba_pkgs?

Originally posted by @jder in #1444 (comment)