Markup nit: use \envvar to mark environment variables.

Doc/whatsnew/whatsnew21.tex

@@ -511,7 +511,7 @@ case-sensitivity on case-insensitive platforms.  Python will now
 search for the first case-sensitive match by default, raising an
 \exception{ImportError} if no such file is found, so \code{import file}
 will not import a module named \samp{FILE.PY}.  Case-insensitive
-matching can be requested by setting the PYTHONCASEOK environment
+matching can be requested by setting the \envvar{PYTHONCASEOK} environment
 variable before starting the Python interpreter.
 
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