bpo-30088: Document that existing dir structure isn't verified by mailbox.Maildir (GH-1163)

Hi,

I've faced an issue w/ `mailbox.Maildir()`. The case is following:
1. I create a folder with `tempfile.TemporaryDirectory()`, so it's empty
2. I pass that folder path as an argument when instantiating `mailbox.Maildir()`
3. Then I receive an exception happening because "there's no such file or directory" (namely `cur`, `tmp` or `new`) during interaction with Maildir

**Expected result:** subdirs are created during `Maildir()` instance creation.

**Actual result:** subdirs are assumed as existing which leads to exceptions during use.

**Workaround:** remove the actual dir before passing the path to `Maildir()`. It will be created automatically with all subdirs needed.

**Fix:** This PR. Basically it adds creation of subdirs regardless of whether the base dir existed before.

https://bugs.python.org/issue30088
(cherry picked from commit e441847)

Co-authored-by: Sviatoslav Sydorenko <wk@sydorenko.org.ua>

Author: miss-islington

Doc/library/mailbox.rst

@@ -308,6 +308,9 @@ Supported mailbox formats are Maildir, mbox, MH, Babyl, and MMDF.
    representation. If *create* is ``True``, the mailbox is created if it does not
    exist.
 
+   If *create* is ``True`` and the *dirname* path exists, it will be treated as
+   an existing maildir without attempting to verify its directory layout.
+
    It is for historical reasons that *dirname* is named as such rather than *path*.
 
    Maildir is a directory-based mailbox format invented for the qmail mail

Misc/ACKS

@@ -1582,6 +1582,7 @@ Kalle Svensson
 Andrew Svetlov
 Paul Swartz
 Al Sweigart
+Sviatoslav Sydorenko
 Thenault Sylvain
 Péter Szabó
 John Szakmeister

Misc/NEWS.d/next/Documentation/2019-07-13-12-43-01.bpo-30088.CIcBjy.rst

@@ -0,0 +1 @@
+Documented that :class:`mailbox.Maildir` constructor doesn't attempt to verify the maildir folder layout correctness. Patch by Sviatoslav Sydorenko.