PYTHON-2431 Fix MONGODB-AWS auth tests on macOS (#521)

(cherry picked from commit 22a7e8085c)

.evergreen/config.yml

@@ -1568,7 +1568,7 @@ axes:
         variables:
           skip_EC2_auth_test: true
           skip_ECS_auth_test: true
-          python3_binary: python3
+          python3_binary: /Library/Frameworks/Python.framework/Versions/3.8/bin/python3
           libmongocrypt_url: https://s3.amazonaws.com/mciuploads/libmongocrypt/macos/master/latest/libmongocrypt.tar.gz
       - id: rhel62
         display_name: "RHEL 6.2 (x86_64)"

.evergreen/run-mongodb-aws-test.sh

@@ -39,7 +39,12 @@ fi
 # show test output
 set -x
 
-VIRTUALENV=$(command -v virtualenv)
+# Workaround macOS python 3.9 incompatibility with system virtualenv.
+if [ $(uname -s) = "Darwin" ]; then
+    VIRTUALENV="/Library/Frameworks/Python.framework/Versions/3.9/bin/python3 -m virtualenv"
+else
+    VIRTUALENV=$(command -v virtualenv)
+fi
 
 authtest () {
     if [ "Windows_NT" = "$OS" ]; then