Improvements to the libmongocrypt Evergreen build (CDRIVER-3124) resulted in build failures (CDRIVER-3138) that have revealed that calc_release_version.py fails on Python 2.6:

https://evergreen.mongodb.com/task/libmongocrypt_rhel_62_64_bit_build_and_test_and_upload_patch_fbb9f59bf32019373232dc1a1fd85a00d6ab95de_5ce414a9850e6169dbb20518_19_05_21_15_09_30

Since RHEL 6 (which has Python 2.6) is a supported/recommended platform for the server product, the C++ Driver build should also work with Python 2.6. This ticket is a clone of CDRIVER-3157 since calc_release_version.py is duplicated between the C and C++ Drivers.