Implement 3.7 features #1965

Let's make sure we have all the new features that will go into 3.7 (going off https://docs.python.org/dev/whatsnew/3.7.html).

• PEP 562 (module __getattr__) doesn't affect typeshed, but may need implementation work in typecheckers
• Changes from PEP 538, 539, 565, 540 don't affect typeshed
• PEP 553 (breakpoint()) was implemented in PEP 553 and PEP 564 #1846.
• PEP 563 (from __future__ import annotations) was implemented in [stdlib/2and3/__future__] Add PEP 563 annotations #1880.
• PEP 564 (time_ns and similar) was implemented in PEP 553 and PEP 564 #1846.
• PEP 557 (dataclasses): active PR Preliminary implementation of stdlib/3.7/dataclasses.pyi. #1944, will need more work in type checkers.
• PEP 567 (contextvars): not yet done.
• https://bugs.python.org/issue30579: TracebackType is instantiable
• new module importlib.resources, and importlib.abc.ResourceReader
• argparse.parse_intermixed_args: Add private members to argparse.pyi. #1937 adds this
• https://bugs.python.org/issue30103: b2a_uu new argument
• https://bugs.python.org/issue30095: new attributes on HTMLCalendar
• contextlib.asynccontextmanager: added by add typing.AsyncContextManager and contextlib.asynccontextmanager #1432
• contextlib.AbstractAsyncContextManager
• contextlib.AsyncExitStack: added by Add stub for AsyncExitContext added by bpo-29302. #1876.
• crypt.mksalt rounds parameter (bpo-31702: Allow to specify rounds for SHA-2 hashing in crypt.mksalt(). cpython#4110)
• datetime.fromisoformat (https://bugs.python.org/issue15873)
• http.client blocksize support (https://bugs.python.org/issue31945)
• http.server directory parameter (https://bugs.python.org/issue28707)
• hmac.digest (https://bugs.python.org/issue32433)
• locale.formatstring monetary argument (https://bugs.python.org/issue10379)
• math.remainder
• os.fwalk bytes paths (https://bugs.python.org/issue28682)
• os.scandir fd support (https://bugs.python.org/issue25996)
• os.register_at_fork (https://bugs.python.org/issue16500)
• pdb.set_trace header argument (https://bugs.python.org/issue31389)
• sqlite3.Connection.backup (https://bugs.python.org/issue27645)
• sys.flags.dev_mode
• tkinter.ttk.SpinBox
• time.CLOCK_* (three added)
• time.thread_time(_ns)? (https://bugs.python.org/issue32025)
• unittest.mock.seal (https://bugs.python.org/issue30541)
• uu.encode backtick argument (https://bugs.python.org/issue30103)
• zipfile.create_archive new arguments (https://bugs.python.org/issue31072, https://bugs.python.org/issue31638)
• some removals (https://docs.python.org/dev/whatsnew/3.7.html#api-and-feature-removals)
  • os.stat_float_times
  • tarfile.Tarfile.add exclude argument
  • ntpath.splitunc
  • verbose argument to collections.NamedTuple
  • plistlib special dicts
  • asyncio.windows_utils.socketpair (no stub for this module in typeshed)
• docstring attribute on some AST nodes (https://bugs.python.org/issue29463)
• struct.Struct.format type change (https://bugs.python.org/issue21071)