Use float value of ST_MTIME in copy_file function

[rajputa-deshaw]

The copy_file function uses st[ST_MTIME] when copying timestamps which is an int
whereas the newer function uses os.path.getmtime which returns a float to check if
a file is outdated. This causes the copied file to always be older than the source file.

This commit updates the copy_file function to use st.st_mtime which is the float value.

Ref: pypa/setuptools#5079

[rajputa-deshaw]

@Avasam Could you please approve the test workflow?

[Avasam]

@Avasam Could you please approve the test workflow?

Whilst I have contributed to, I am not a maintainer of distutils (or setuptools). I cannot approve PRs or workflows.

[rajputa-deshaw]

@jaraco Could you review this PR?

[rajputa-deshaw]

Actually, we should use os.utime(dst, ns=(st.st_atime_ns, st.st_mtime_ns)) for full precision.