This is a collection of no-op statements that are syntactically valid only since Python X.Y, for most X.Y ≥ 2.2.

For example:

0_0  # Python >= 3.6 is required

The idea is to put such a statement near the top of your file. If a user inadvertently ran the code against an older version, they would get a fairly helpful error message:

$ python3.5 since-3.6.py
  File "since-3.6.py", line 1
    0_0  # Python >= 3.6 is required
      ^
SyntaxError: invalid syntax