Fix scanner - ignore trailing tabs

[eihli]

Trying to address this TODO:

TODO: We need to make tab handling rules more sane. A good rule is
Tabs cannot precede tokens
BLOCK-SEQUENCE-START, BLOCK-MAPPING-START, BLOCK-END,
KEY(block), VALUE(block), BLOCK-ENTRY

I don't quite understand the "Tabs cannot precede tokens" statement though. My understanding of the YAML spec is that tabs can be treated the exact same as spaces - before/after token doesn't matter.

But the change I made here took to heart the "Tabs cannot precede tokens" statement and only ignores tabs when they directly precede other tabs/spaces followed by a newline.

Before:

(ghsync) ➜  pyyaml git:(fix-tabs) ✗ python setup.py test
running test
# ...
  File "/var/folders/p0/9kyh1lx53tb5_9tsft21ffrc0000gn/T/test_pyyaml89f6sqhb/pyyaml/yaml/scanner.py", line 258, in fetch_more_tokens
    raise ScannerError("while scanning for the next token", None,
yaml.scanner.ScannerError: while scanning for the next token
found character '\t' that cannot start any token
  in "tests/data/trailing-tab.data", line 2, column 8
---------------------------------------------------------------------------
tests/data/trailing-tab.data:
foo:
  - bar
---------------------------------------------------------------------------
tests/data/trailing-tab.code:
{"foo": ["bar"]}
===========================================================================
TESTS: 1285
ERRORS: 1
error: Tests failed

[eihli]

Anyone know of who I can ping to get traction on this? I just ran into an issue again today.

[eihli]

Thanks. I'll sign this commit tonight.

[eihli]

@bitcoinfinancier Commit signed.