Stop using IO::NULL for feature detection

`IO::NULL`'s underlying file `/dev/null` is not always available on
WASI, so use of the file on top-level code (introduced in
#36) causes tempfile library not
to work at all on WASI.

Author: kateinoigakukun

lib/tempfile.rb

@@ -593,9 +593,7 @@ class << Tempfile
   end
 end
 
-File.open(IO::NULL) do |f|
-  File.new(f.fileno, autoclose: false, path: "").path
-rescue IOError
+if RUBY_VERSION < "3.2"
   module PathAttr               # :nodoc:
     attr_reader :path