[#3583] Fix encoding error with print statements in doctests

This fix was suggested by Stack Overflow user phd in
<https://stackoverflow.com/a/50863820/744178>.

Author: jwodder

changelog/3583.bugfix.rst

@@ -0,0 +1 @@
+Fix encoding error with `print` statements in doctests

src/_pytest/doctest.py

@@ -505,7 +505,7 @@ class UnicodeSpoof(_SpoofOut):
 
         def getvalue(self):
             result = _SpoofOut.getvalue(self)
-            if encoding:
+            if encoding and isinstance(result, bytes):
                 result = result.decode(encoding)
             return result