Replace usage of os.devnull with subprocess.DEVNULL (#13932)

Refactoring one file in the latex library

Closes https://github.com/ipython/ipython/issues/13906

# Code changes

Replaced one occurence of `open(os.devnull)` with using directly the
`subprocess.DEVNULL` special value, removing a with/open block along the
way.

# User-facing changes

None

IPython/lib/latextools.py

@@ -160,12 +160,11 b" def latex_to_png_dvipng(s, wrap, color='Black', scale=1.0):"
         with workdir.joinpath(tmpfile).open("w", encoding="utf8") as f:
             f.writelines(genelatex(s, wrap))
 
-        with open(os.devnull, 'wb') as devnull:
             subprocess.check_call(
                 ["latex", "-halt-on-error", "-interaction", "batchmode", tmpfile],
                 cwd=workdir,
-                stdout=devnull,
-                stderr=devnull,
+            stdout=subprocess.DEVNULL,
+            stderr=subprocess.DEVNULL,
                 startupinfo=startupinfo,
             )
 
@@ -188,8 +187,8 b" def latex_to_png_dvipng(s, wrap, color='Black', scale=1.0):"
                     color,
                 ],
                 cwd=workdir,
-                stdout=devnull,
-                stderr=devnull,
+            stdout=subprocess.DEVNULL,
+            stderr=subprocess.DEVNULL,
                 startupinfo=startupinfo,
             )