subprocess: make env accept None also on python2 (#4504)

This was made for python3 in https://github.com/python/typeshed/pull/1291 but
this is also the case on python2

stdlib/2/subprocess.pyi

@@ -21,7 +21,7 @@ def call(
     close_fds: bool = ...,
     shell: bool = ...,
     cwd: Optional[_TXT] = ...,
-    env: _ENV = ...,
+    env: Optional[_ENV] = ...,
     universal_newlines: bool = ...,
     startupinfo: Any = ...,
     creationflags: int = ...,
@@ -37,7 +37,7 @@ def check_call(
     close_fds: bool = ...,
     shell: bool = ...,
     cwd: Optional[_TXT] = ...,
-    env: _ENV = ...,
+    env: Optional[_ENV] = ...,
     universal_newlines: bool = ...,
     startupinfo: Any = ...,
     creationflags: int = ...,
@@ -54,7 +54,7 @@ def check_output(
     close_fds: bool = ...,
     shell: bool = ...,
     cwd: Optional[_TXT] = ...,
-    env: _ENV = ...,
+    env: Optional[_ENV] = ...,
     universal_newlines: bool = ...,
     startupinfo: Any = ...,
     creationflags: int = ...,