mass rewrite of string formatting to use f-strings everywhere

performed by running "pyupgrade --py36-plus" and committing the results
diff --git a/skip_ci.py b/skip_ci.py
index 4ee4466..7411d57 100755
--- a/skip_ci.py
+++ b/skip_ci.py
@@ -23,16 +23,16 @@
 
 def check_pr(is_pr_env):
     if is_pr_env not in os.environ:
-        print('This is not pull request: {} is not set'.format(is_pr_env))
+        print(f'This is not pull request: {is_pr_env} is not set')
         sys.exit()
     elif os.environ[is_pr_env] == 'false':
-        print('This is not pull request: {} is false'.format(is_pr_env))
+        print(f'This is not pull request: {is_pr_env} is false')
         sys.exit()
 
 
 def get_base_branch(base_env):
     if base_env not in os.environ:
-        print('Unable to determine base branch: {} is not set'.format(base_env))
+        print(f'Unable to determine base branch: {base_env} is not set')
         sys.exit()
     return os.environ[base_env]