[lit] Remove StringIO imports

StringIO was moved to io.StringIO in Python 3. These try/catch
statements were intended to provide cross compatibility between the two.
Now that LLVM's minimum required Python version is 3.8 and Python 2 has
been deprecated for a while, we can probably drop support. Especially
since there is no upstream testing for this configuration.

Author: boomanaiden154

lit/TestRunner.py

@@ -15,13 +15,7 @@
 import typing
 import traceback
 from typing import Optional, Tuple
-
-import io
-
-try:
-    from StringIO import StringIO
-except ImportError:
-    from io import StringIO
+from io import StringIO
 
 from lit.ShCommands import GlobItem, Command
 import lit.ShUtil as ShUtil

lit/builtin_commands/cat.py

@@ -1,10 +1,6 @@
 import getopt
 import sys
-
-try:
-    from StringIO import StringIO
-except ImportError:
-    from io import StringIO
+from io import StringIO
 
 
 def convertToCaretAndMNotation(data):