Graceful exit on KeyboardInterrupt (#10725)

Catch KeyboardInterrupt to allow graceful exit instead of printing the traceback.

Adds options parsing to the KeyboardInterrupt handler. At the moment
the traceback will only be printed on --show-traceback but can easily 
extend it to -v as well.

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

mypy/__main__.py

@@ -1,9 +1,10 @@
 """Mypy type checker command line tool."""
-
-import sys
 import os
+import sys
+import traceback
 
-from mypy.main import main
+from mypy.main import main, process_options
+from mypy.util import FancyFormatter
 
 
 def console_entry() -> None:
@@ -17,6 +18,16 @@ def console_entry() -> None:
         devnull = os.open(os.devnull, os.O_WRONLY)
         os.dup2(devnull, sys.stdout.fileno())
         sys.exit(2)
+    except KeyboardInterrupt:
+        _, options = process_options(args=sys.argv[1:])
+        if options.show_traceback:
+            sys.stdout.write(traceback.format_exc())
+        formatter = FancyFormatter(sys.stdout, sys.stderr, False)
+        msg = "Interrupted\n"
+        sys.stdout.write(formatter.style(msg, color="red", bold=True))
+        sys.stdout.flush()
+        sys.stderr.flush()
+        sys.exit(2)
 
 
 if __name__ == '__main__':