Issue #8513: os.get_exec_path() supports b'PATH' key and bytes value.
subprocess.Popen() and os._execvpe() support bytes program name. Add
os.supports_bytes_environ flag: True if the native OS type of the environment
is bytes (eg. False on Windows).
diff --git a/Doc/library/os.rst b/Doc/library/os.rst
index 1d4b1e1..fbc7edb 100644
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -142,7 +142,8 @@
    synchronized (modify :data:`environb` updates :data:`environ`, and vice
    versa).
 
-   Availability: Unix.
+   :data:`environb` is only available if :data:`supports_bytes_environ` is
+   True.
 
    .. versionadded:: 3.2
 
@@ -457,6 +458,12 @@
    Availability: Unix, Windows.
 
 
+.. data:: supports_bytes_environ
+
+   True if the native OS type of the environment is bytes (eg. False on
+   Windows).
+
+
 .. function:: umask(mask)
 
    Set the current numeric umask and return the previous umask.