cmdutil: stop using the `pycompat.open()` shim

Somewhat dubious I suppose, but maybe it makes sense to use the type checking
system to more aggressively use str for trivial things like the `mode` argument
to the function that calls this.  That's a project for another day.

mercurial/cmdutil.py

@@ -30,9 +30,6 from .node import (
     nullrev,
     short,
 )
-from .pycompat import (
-    open,
-)
 from .thirdparty import attr
 
 # Force pytype to use the non-vendored package
@@ -1406,7 +1403,7 def makefileobj(ctx, pat, mode=b'wb', **
             fp = repo.ui.fin
         return _unclosablefile(fp)
     fn = makefilename(ctx, pat, **props)
-    return open(fn, mode)
+    return open(fn, pycompat.sysstr(mode))
 
 
 def openstorage(repo, cmd, file_, opts, returnrevlog=False):