3.11.0b4: zero line numbers

[nedbat]

Describe the bug
CPython 3.11.0b4 has bytecodes with line number zero. This adds a line that cannot be covered, affecting coverage outputs.

The CPython commit that introduced the problem was python/cpython@1bfe83a.

[nedbat]

This is now released as part of coverage 6.4.2.