Add functionality for auto-revoking auth tokens

[jmoldow]

Auth objects can now be closed, which prevents them from being
used to request new tokens. This will also revoke any existing
tokens. Also introduces a closing() context manager method,
which will auto-close the auth object on exit. Clients can use
this to make sure that their tokens don't live longer than they
need them for.

[boxcla]

Verified that @jmoldow has signed the CLA. Thanks for the pull request!

[Jeff-Meadows]

Should we add an auto-revoking client that uses this auth behavior?

[Jeff-Meadows]

I'm not sure ValueError is correct here. I'd raise RuntimeError or a custom Exception

[jmoldow]

ValueError is what the standard library uses for similar purposes:

jmoldow Lib ((detached from v3.6.2)) $ git grep -C9 "if .*closed.*[:]" | grep -v asyncio | grep -v pyio | grep -v "test" | grep --color=always "  raise "
_compression.py-14-            raise ValueError("I/O operation on closed file")
_compression.py-18-            raise io.UnsupportedOperation("File not open for reading")
_compression.py-22-            raise io.UnsupportedOperation("File not open for writing")
chunk.py-95-            raise ValueError("I/O operation on closed file")
chunk.py-105-            raise ValueError("I/O operation on closed file")
chunk.py-107-            raise OSError("cannot seek")
chunk.py-113-            raise RuntimeError
chunk.py-119-            raise ValueError("I/O operation on closed file")
chunk.py-129-            raise ValueError("I/O operation on closed file")
chunk.py-153-            raise ValueError("I/O operation on closed file")
idlelib/run.py-339-            raise ValueError("write to closed file")
idlelib/run.py-342-                raise TypeError('must be str, not ' + type(s).__name__)
idlelib/run.py-359-            raise ValueError("read from closed file")
idlelib/run.py-363-            raise TypeError('must be int, not ' + type(size).__name__)
idlelib/run.py-382-            raise ValueError("read from closed file")
idlelib/run.py-386-            raise TypeError('must be int, not ' + type(size).__name__)
pathlib.py-1011-        raise ValueError("I/O operation on closed path")
pathlib.py-1072-            raise ValueError("Unacceptable pattern: {!r}".format(pattern))
pathlib.py-1191-            raise TypeError('data must be str, not %s' %
socket.py-615-            raise ValueError("I/O operation on closed socket.")
socket.py-622-            raise ValueError("I/O operation on closed socket.")
socket.py-629-            raise ValueError("I/O operation on closed socket.")
subprocess.py-188-            raise ValueError("already closed")
tarfile.py-408-                raise CompressionError("unknown compression type %r" % comptype)
tarfile.py-2365-            raise OSError("%s is closed" % self.__class__.__name__)
tarfile.py-2367-            raise OSError("bad operation for mode %r" % self.mode)
tempfile.py-678-            raise ValueError("Cannot enter context with closed file")
urllib/response.py-30-            raise ValueError("I/O operation on closed file")
zipfile.py-985-            raise ValueError('I/O operation on closed file.')

[jmoldow]

@Jeff-Meadows see my comment in #175. I experimented with adding a __del__ for OAuth2, but I didn't like the behavior, so I removed it.

For the client, we could think about making it into a context manager, which invoke's auth.closing().