Use python3 for linters

We now require python3 for the testsuite so rather than require two
versions of python it makes sense to use python3 for the linters as
well.

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2888

.arc-linters/check-binaries.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # A linter to warn when binary files are added to the repository
 

.arc-linters/check-cpp.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # A linter to warn for ASSERT macros which are separated from their argument
 # list by a space, which Clang's CPP barfs on

.arc-linters/check-makefiles.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Warn for use of `--interactive` inside Makefiles (#11468).
 #

.arclint

@@ -71,17 +71,17 @@
     },
     "check-binaries": {
       "type": "external-json",
-      "external-json.script": "python .arc-linters/check-binaries.py"
+      "external-json.script": "python3 .arc-linters/check-binaries.py"
     },
     "check-makefiles": {
       "type": "external-json",
       "include": ["(Makefile$)"],
-      "external-json.script": "python .arc-linters/check-makefiles.py"
+      "external-json.script": "python3 .arc-linters/check-makefiles.py"
     },
     "bad-assert-clang-cpp": {
       "type": "external-json",
       "include": ["(\\.(l?hs|x|y\\.pp)(\\.in)?$)", "(\\.(c|h)$)"],
-      "external-json.script": "python .arc-linters/check-cpp.py"
+      "external-json.script": "python3 .arc-linters/check-cpp.py"
     }
   },