handle version calculation on systems without python2

Author: rcsanchez97

.evergreen/config.yml

@@ -23,7 +23,12 @@ functions:
         set -o errexit
         if [ -n "${github_pr_number}" -o "${is_patch}" = "true" ]; then
            # This is a GitHub PR or patch build, probably branched from master
-           echo $(python ./build/calc_release_version.py --next-minor) > VERSION_CURRENT
+           if command -v python3 2>/dev/null; then
+              # Prefer python3 if it is available
+              echo $(python3 ./build/calc_release_version.py --next-minor) > VERSION_CURRENT
+           else
+              echo $(python ./build/calc_release_version.py --next-minor) > VERSION_CURRENT
+           fi
            VERSION=$VERSION_CURRENT-${version_id}
         else
            VERSION=latest

build/evergreen_config_lib/functions.py

@@ -29,7 +29,12 @@
         shell_mongoc(r'''
         if [ -n "${github_pr_number}" -o "${is_patch}" = "true" ]; then
            # This is a GitHub PR or patch build, probably branched from master
-           echo $(python ./build/calc_release_version.py --next-minor) > VERSION_CURRENT
+           if command -v python3 2>/dev/null; then
+              # Prefer python3 if it is available
+              echo $(python3 ./build/calc_release_version.py --next-minor) > VERSION_CURRENT
+           else
+              echo $(python ./build/calc_release_version.py --next-minor) > VERSION_CURRENT
+           fi
            VERSION=$VERSION_CURRENT-${version_id}
         else
            VERSION=latest