Fix import formatting per lintrunner

- Add blank line between stdlib and third-party imports
- Sort imports alphabetically

Author: seemethere

test/land/dry_run.py.test

@@ -2,7 +2,8 @@ from ghstack.test_prelude import *
 
 import os
 import tempfile
-from ghstack.github_fake import GitHubNumber, PullRequestReview, CheckRun
+
+from ghstack.github_fake import CheckRun, GitHubNumber, PullRequestReview
 
 init_test()
 commit("A")

test/land/validate_rules_fail.py.test

@@ -2,6 +2,7 @@ from ghstack.test_prelude import *
 
 import os
 import tempfile
+
 import ghstack.merge_rules
 from ghstack.github_fake import GitHubNumber
 

test/land/validate_rules_pass.py.test

@@ -2,7 +2,8 @@ from ghstack.test_prelude import *
 
 import os
 import tempfile
-from ghstack.github_fake import GitHubNumber, PullRequestReview, CheckRun
+
+from ghstack.github_fake import CheckRun, GitHubNumber, PullRequestReview
 
 init_test()
 commit("A")