Use python 3.9 in conda osx build.

Author: 0mdc

.circleci/config.yml

@@ -312,7 +312,8 @@ jobs:
           command: |
             export PATH=$HOME/miniconda/bin:$PATH
             source ~/.bash_profile
-            conda activate
+            conda create --name py39 python=3.9 -y
+            conda activate py39
             conda install -y anaconda-client ccache cmake git ninja conda-build pip
             echo $(which -a python)
             pip install gitpython