Removing uses of env.PY2 and env.PY (no longer available).

smart_holder_type_casters.h tweaks:
    * Removing MSVC 2015 workaround.
    * Manually adding braces to pre-empt clang-tidy issue (involving `#ifdef`).
    * Removing `// clang-format off` & `on`.
    * One manual line break (moving comment above line).

Adding mypy type information for ubench/*.py