lib/open3.rb: make sure that pipes are closed

Attempting to invoke a non-existent command led to the leak of fds.

http://rubyci.s3.amazonaws.com/graviton2/ruby-master/log/20200526T140004Z.log.html.gz
```
[ 9101/20195] TestGemExtCmakeBuilder#test_self_build = 0.01 sLeaked file descriptor: TestGemExtCmakeBuilder#test_self_build: 7 : #<IO:fd 7>
Leaked file descriptor: TestGemExtCmakeBuilder#test_self_build: 10 : #<IO:fd 10>
Leaked file descriptor: TestGemExtCmakeBuilder#test_self_build: 11 : #<IO:fd 11>
Leaked file descriptor: TestGemExtCmakeBuilder#test_self_build: 12 : #<IO:fd 12>
```

lib/open3.rb

@@ -206,6 +206,11 @@ module Open3
     opts[[:out, :err]] = out_w
 
     popen_run(cmd, opts, [in_r, out_w], [in_w, out_r], &block)
+  ensure
+    in_r.close
+    in_w.close
+    out_r.close
+    out_w.close
   end
   module_function :popen2e