Fix deprecations in Python 3.7.

nmigen/compat/fhdl/module.py

@@ -1,4 +1,4 @@
-from collections import Iterable
+from collections.abc import Iterable
 
 from ...tools import flatten, deprecated
 from ...fhdl import dsl

nmigen/fhdl/dsl.py

@@ -1,4 +1,5 @@
-from collections import OrderedDict, Iterable
+from collections import OrderedDict
+from collections.abc import Iterable
 from contextlib import contextmanager
 
 from .ast import *

nmigen/fhdl/xfrm.py

@@ -1,4 +1,5 @@
-from collections import OrderedDict, Iterable
+from collections import OrderedDict
+from collections.abc import Iterable
 
 from ..tools import flatten
 from .ast import *

nmigen/tools.py

@@ -1,4 +1,4 @@
-from collections import Iterable
+from collections.abc import Iterable
 import functools
 import warnings