Ubuntu 18 no longer has Python 2.7, 3.4, or 3.5 installed on it in the latest version of the toolchain. We need to migrate testing to amazon1-2018.

https://evergreen.mongodb.com/build/mongo_python_driver_3.12_tests_python_version_ubuntu18_compression__platform~ubuntu_18.04_compression~snappy_python_version~2.7_c_extensions~with_c_extensions_8455b764fd6b5e2919f9e1f6c3160a3f19ae82e7_21_06_24_19_37_58