.gitattributes

convert: drop a duplicate implementation of `dateutil.makedate()`

I noticed this because the signature generated by pytype recently changed to be
less specific.  When the method was introduced back in 337d728e644f,
`util.makedate()` didn't take an optional timestamp arg.  But now it does, and
the methods are the same (except the `dateutil` version validates that the
timestamp isn't a negative value).  I left the old method in place in case
anyone has custom convert code that monkey patches it.

# So GitLab doesn't think we're using tons of Perl
*.t -linguist-detectable