[lit] Remove python 2.7 code paths in builtin diff

Lit's builtin diff command had some Python 2.7 code paths lying around
that we can probably get rid of at this point. LLVM at this point
requires Python 3.8 at minimum. Keeping lit working at a lower version
is a reasonable goal, but I think we can probably drop python 2 support
at this point given how long it has been deprecated and how long LLVM
has supported Python 3.

Reviewers: jdenny-ornl, ilovepi, petrhosek

Reviewed By: ilovepi

Pull Request: #157558

Author: boomanaiden154

llvm/docs/ReleaseNotes.md

@@ -152,6 +152,7 @@ Changes to the LLVM tools
 ---------------------------------
 
 * `llvm-readelf` now dumps all hex format values in lower-case mode.
+* Some code paths for supporting Python 2.7 in `llvm-lit` have been removed.
 
 Changes to LLDB
 ---------------------------------

llvm/utils/lit/lit/builtin_commands/diff.py

@@ -59,30 +59,15 @@ def compareTwoFiles(flags, filepaths):
 
 def compareTwoBinaryFiles(flags, filepaths, filelines):
     exitCode = 0
-    if hasattr(difflib, "diff_bytes"):
-        # python 3.5 or newer
-        diffs = difflib.diff_bytes(
-            difflib.unified_diff,
-            filelines[0],
-            filelines[1],
-            filepaths[0].encode(),
-            filepaths[1].encode(),
-            n=flags.num_context_lines,
-        )
-        diffs = [diff.decode(errors="backslashreplace") for diff in diffs]
-    else:
-        # python 2.7
-        if flags.unified_diff:
-            func = difflib.unified_diff
-        else:
-            func = difflib.context_diff
-        diffs = func(
-            filelines[0],
-            filelines[1],
-            filepaths[0],
-            filepaths[1],
-            n=flags.num_context_lines,
-        )
+    diffs = difflib.diff_bytes(
+        difflib.unified_diff,
+        filelines[0],
+        filelines[1],
+        filepaths[0].encode(),
+        filepaths[1].encode(),
+        n=flags.num_context_lines,
+    )
+    diffs = [diff.decode(errors="backslashreplace") for diff in diffs]
 
     for diff in diffs:
         sys.stdout.write(to_string(diff))
@@ -230,14 +215,8 @@ def compareDirTrees(flags, dir_trees, base_paths=["", ""]):
 
 def main(argv):
     if sys.platform == "win32":
-        if hasattr(sys.stdout, "buffer"):
-            # python 3
-            sys.stdout = io.TextIOWrapper(sys.stdout.buffer, newline="\n")
-        else:
-            # python 2.7
-            import msvcrt
+        sys.stdout = io.TextIOWrapper(sys.stdout.buffer, newline="\n")
 
-            msvcrt.setmode(sys.stdout.fileno(), os.O_BINARY)
     args = argv[1:]
     try:
         opts, args = getopt.gnu_getopt(args, "wbuI:U:r", ["strip-trailing-cr"])