Fail to parse document containing trailing tab in line

[phil9909]

#!/usr/bin/env python3

import yaml
document = """
a:\t
  - 1
"""
print(yaml.dump(yaml.load(document)))

produces the following output when running with pyyaml 5.1.1

__main__:1: YAMLLoadWarning: calling yaml.load() without Loader=... is deprecated, as the default Loader is unsafe. Please read https://msg.pyyaml.org/load for full details.
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.7/site-packages/yaml/__init__.py", line 114, in load
    return loader.get_single_data()
  File "/usr/local/lib/python3.7/site-packages/yaml/constructor.py", line 41, in get_single_data
    node = self.get_single_node()
  File "/usr/local/lib/python3.7/site-packages/yaml/composer.py", line 36, in get_single_node
    document = self.compose_document()
  File "/usr/local/lib/python3.7/site-packages/yaml/composer.py", line 55, in compose_document
    node = self.compose_node(None, None)
  File "/usr/local/lib/python3.7/site-packages/yaml/composer.py", line 84, in compose_node
    node = self.compose_mapping_node(anchor)
  File "/usr/local/lib/python3.7/site-packages/yaml/composer.py", line 133, in compose_mapping_node
    item_value = self.compose_node(node, item_key)
  File "/usr/local/lib/python3.7/site-packages/yaml/composer.py", line 64, in compose_node
    if self.check_event(AliasEvent):
  File "/usr/local/lib/python3.7/site-packages/yaml/parser.py", line 98, in check_event
    self.current_event = self.state()
  File "/usr/local/lib/python3.7/site-packages/yaml/parser.py", line 449, in parse_block_mapping_value
    if not self.check_token(KeyToken, ValueToken, BlockEndToken):
  File "/usr/local/lib/python3.7/site-packages/yaml/scanner.py", line 116, in check_token
    self.fetch_more_tokens()
  File "/usr/local/lib/python3.7/site-packages/yaml/scanner.py", line 260, in fetch_more_tokens
    self.get_mark())
yaml.scanner.ScannerError: while scanning for the next token
found character '\t' that cannot start any token
  in "<unicode string>", line 2, column 3:
    a:
      ^

Expected behaviour

The tab-character is ignored (like space) as reference parser does http://ben-kiki.org/ypaste/data/1493/index.html and when using Loader=yaml.CLoader