Revert "pyupgrade --py37-plus **/*.py"

This reverts commit 12f8cbe.

This commit introduced a series of unit test failures, although
I'm not completely sure why.

backoff/__init__.py

@@ -1,3 +1,4 @@
+# coding:utf-8
 """
 Function decoration for backoff and retry
 

backoff/_async.py

@@ -1,3 +1,4 @@
+# coding:utf-8
 import datetime
 import functools
 import asyncio

backoff/_common.py

@@ -1,3 +1,5 @@
+# coding:utf-8
+
 import functools
 import logging
 import sys

backoff/_decorator.py

@@ -1,3 +1,4 @@
+# coding:utf-8
 import asyncio
 import logging
 import operator

backoff/_jitter.py

@@ -1,3 +1,5 @@
+# coding:utf-8
+
 import random
 
 

backoff/_sync.py

@@ -1,3 +1,4 @@
+# coding:utf-8
 import datetime
 import functools
 import time

backoff/_typing.py

@@ -1,3 +1,4 @@
+# coding:utf-8
 import logging
 import sys
 from typing import (Any, Callable, Coroutine, Dict, Generator, Sequence, Tuple,

backoff/_wait_gen.py

@@ -1,3 +1,5 @@
+# coding:utf-8
+
 import itertools
 from typing import Any, Callable, Generator, Iterable, Optional, Union
 
@@ -66,7 +68,8 @@ def constant(
     except TypeError:
         itr = itertools.repeat(interval)  # type: ignore
 
-    yield from itr
+    for val in itr:
+        yield val
 
 
 def runtime(

backoff/types.py

@@ -1,3 +1,4 @@
+# coding:utf-8
 from ._typing import Details
 
 __all__ = [

tests/common.py

@@ -1,3 +1,4 @@
+# coding:utf-8
 import collections
 import functools
 

tests/test_backoff.py

@@ -1,3 +1,4 @@
+# coding:utf-8
 import datetime
 import itertools
 import logging

tests/test_backoff_async.py

@@ -1,3 +1,5 @@
+# coding:utf-8
+
 import asyncio  # Python 3.5 code and syntax is allowed in this file
 import random
 

tests/test_jitter.py

@@ -1,3 +1,4 @@
+# coding:utf-8
 import backoff
 
 

tests/test_types.py

@@ -1,3 +1,5 @@
+# coding:utf-8
+
 from backoff.types import Details
 
 

tests/test_wait_gen.py

@@ -1,3 +1,4 @@
+# coding:utf-8
 import backoff