Strings with question marks get quotes removed

[lyz-code]

I have the following yaml file:

---
- key: ['value?']

which loads correctly in python with pyyaml's yaml.safe_load.

ruyaml changes the file to

---
- key: [value?]

which cannot be loaded by pyyaml's yaml.safe_load, error being "expected ',' or ']', but got '?'"

yq breaks similarly with error parse error: Invalid numeric literal at line 2, column 0

It would be nice if ruyaml did not remove quotes on strings with question marks (and whatever other set of characters can cause this issue).

I've assembled a small example to reproduce the issue:

from io import StringIO

import ruyaml

original_source_code = "---\n - key: ['value?']"

yaml = ruyaml.main.YAML()
yaml.indent(mapping=2, sequence=4, offset=2)
yaml.allow_duplicate_keys = True
yaml.explicit_start = True
yaml.width = 80

source_dicts = yaml.load_all(original_source_code)
string_stream = StringIO()
for source_dict in source_dicts:
    yaml.dump(source_dict, string_stream)
    source_code = string_stream.getvalue()
string_stream.close()

assert original_source_code == source_code
>>> AssertionError
>>> source_code
'---\n  - key: [value?]\n'

Thank you

Related issue: lyz-code/yamlfix#188