return flat list from derived object's formulate() (#629)

The StdinDataRedirection command object was returning a list inside a
list, which caused trouble when getting the "str()" of the command.

Example:

    Python 3.10.8 (tags/v3.10.8:aaaf517, Oct 11 2022, 16:50:30) [MSC v.1933 64 bit (AMD64)] on win32
    Type "help", "copyright", "credits" or "license" for more information.
    >>> from plumbum.cmd import head
    >>> cmd = head['-2'] << "one\ntwo\nthree\nfour"
    >>> print(f"{cmd}")
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "C:\Python310\lib\site-packages\plumbum\commands\base.py", line 66, in __str__
        return " ".join(self.formulate())
    TypeError: sequence item 2: expected str instance, list found
    >>>

Co-authored-by: Jason Singer (js731079) <[email protected]>

plumbum/commands/base.py

@@ -533,9 +533,8 @@ def _get_encoding(self):
     def formulate(self, level=0, args=()):
         return [
             f"echo {shquote(self.data)}",
-            "|",
-            self.cmd.formulate(level + 1, args),
-        ]
+            "|"
+        ] + self.cmd.formulate(level + 1, args),
 
     @property
     def machine(self):