Revert "Deprecates RUN_THIS.py in favor of manually doin' it (#3930)" (#3933)

This reverts commit 94082e07a7.

RUN_THIS.py

@@ -0,0 +1,18 @@
+#!/usr/bin/env python3
+
+# Import future so people on py2 still get the clear error that they need to upgrade.
+from __future__ import print_function
+import sys
+import subprocess
+
+IS_WINDOWS = sys.platform in ("win32", "cygwin")
+
+version = sys.version_info
+if version.major < 3 or (version.major == 3 and version.minor < 5):
+    print("ERROR: You need at least Python 3.5 to build SS14.")
+    sys.exit(1)
+
+if IS_WINDOWS:
+    subprocess.run(["py", "-3", "git_helper.py"], cwd="BuildChecker")
+else:
+    subprocess.run(["python3", "git_helper.py"], cwd="BuildChecker")