If a .py file changes multiple times we can end up in a situation
where there is an .py file with corresponding .pyc file that we
rewrote, so both are now mtime==1. Then a new version of the .py file
is added, but the corresponding .pyc file is not updated. This means
that the .pyc file is stale, and python would not normally use it.
However, we will later change the mtime on the .py file to 1, causing
the old .pyo file to look up-to-date even though its stale.
We fix this by detecting the case where the is a new mtime on a .py file
where the .pyc file doesn't match, and remove the stale .pyc file.
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