Replace uses of sphinx.locale.l_ with sphinx.locale._
Replace uses of sphinx.locale.l_ with sphinx.locale._

l_ was deprecated and ultimately removed.  The deprecation message says to use
_ instead, so do that.

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Signed-off-by: Lydia Duncan <f955cf5f42dff0db8f444c4740a67a0cf0b4805d@users.noreply.github.com>

chapel-lang/sphinxcontrib-chapeldomain