Do not auto-create .taskrc when stdout is not a TTY (#3888)

* Do not auto-create .taskrc when stdout is not a TTY

This avoids prompting or automatically creating such a file, both of
which are unexpected when performing command-line completion.

Fixes #3751.

* Test case for taskrc creation no longer works

A taskrc is only created when stdout is a tty, which would require
allocating a pty, which is very platform-dependent and definitely not
worth the trouble for this test.

test/taskrc.test.py

@@ -40,6 +40,7 @@ class TestTaskrc(TestCase):
         """Executed before each test in the class"""
         self.t = Task()
 
+    @unittest.skip("taskrc generation requires a tty - see #3751")
     def test_default_taskrc(self):
         """Verify that a default .taskrc is generated"""
         os.remove(self.t.taskrc)