asyncio doc: add "asyncio-" prefix to references
whatsnew: fix importlib.reload entry.

Turns out I committed a work-in-progress entry because of a time
gap between when I wrote it and when I committed.
whatsnew: filecmp.clear_cache, and reword description of cache in docs.
whatsnew: some more importlib replacements for imp functions.

get_magic->util.MAGIC_NUMBER, source_from_cache, and cache_from_source.
inspect.signature: Use 'inspect.isbuiltin' in 'Signature.from_builtin'
whatsnew: -m <namespace package>, plus 'using' doc updates.
whatsnew: hmac accepts more bytes types, importlib decode_source, stat in C.
use with statement to ensure zipfile is always closed (closes #20102)
merge 3.3 (#20102)
Issue #19320: Fixed split/splitlist tests in test_tcl for Tcl 8.5.0-8.5.5.
Issue #19320: Fixed split/splitlist tests in test_tcl for Tcl 8.5.0-8.5.5.
asyncio: document the new asyncio.subprocess module
Issue #20423: fix documentation of io.StringIO's newline parameter
Issue #20423: fix documentation of io.StringIO's newline parameter
Issue #19990: Install test/imghdrdata.
Issue #19990: Install test/imghdrdata.
Issue #20435: Fix _pyio.StringIO.getvalue() to take into account newline translation settings.
Issue #20435: Fix _pyio.StringIO.getvalue() to take into account newline translation settings.
Issue #20472: test_asyncio: skip PTY tests on Mac OS X older than 10.6
Issue #20472: asyncio: Adjust the note about Mac OS X on PTY, specify that it
requires at least Mac OS X 10.6.