Patch by Adrian von Bidder.

In order to create symlinks on Windows, SeCreateSymbolicLinkPrivilege
is an account privilege that is required to be held by the user. Not only
must the privilege be enabled for the account, the activated privileges for
the currently running application must be adjusted to enable the requested
privilege.

Rather than exposing an additional function to be called prior to the user's
first os.symlink call, we handle the AdjustTokenPrivileges Windows API call
internally and only expose os.symlink when the privilege escalation was
successful.

Due to the change of only exposing os.symlink when it's available, we can
go back to the original test skipping methods of checking via `hasattr`.

#7475: add (un)transform method to bytes/bytearray and str, add back codecs that can be used with them from Python 2.

#10597: fix Py_SetPythonHome docs by pointing to where the meaning of PYTHONHOME is already documented.

Issue 9299 Add exist_ok parameter to os.makedirs to suppress 'File exists' exception. Patch by Ray Allen.

Issue #9573: os.fork now works when triggered as a side effect of import (the wisdom of actually relying on this remains questionable!)

Patch by Xuanji Li.

Split a common setUp/tearDown into the appropriate parts.

much larger than operand `b`.  Patch by Andrew Bennetts.