Spaces at start/end of a sequence

[dga-nagra]

Issue Description

Dumping a flow sequence does not allow much control over the spacing.

I can for example generate this sequence:

[item1, item2, item3]

but this is not compliant with a possible yamllint configuration like this one:

...
  brackets:
    min-spaces-inside: 1
    max-spaces-inside: 1
    min-spaces-inside-empty: 0
    max-spaces-inside-empty: 0

We need to be able to generate the following

[ item1, item2, item3 ]

---

Workaround

I am currently inheriting the Emitter class and injecting it in my YAML instance

import ruamel.yaml as yaml
from ruamel.yaml.emitter import Emitter as BaseEmitter
from ruamel.yaml.events import SequenceEndEvent

# This inheritance is made in order to add leading and trealing spaces in sequences
# [item] -> [ item ]
class Emitter(BaseEmitter):
    def expect_first_flow_sequence_item(self) -> None:
        if isinstance(self.event, SequenceEndEvent):
            self.indent = self.indents.pop()
            popped = self.flow_context.pop()
            assert popped == "["  # noqa
            self.write_indicator(self.flow_seq_end, True)  # EDITED  <-----------------------------------------------
            if self.event.comment and self.event.comment[0]:
                # eol comment on empty flow sequence
                self.write_post_comment(self.event)
            elif self.flow_level == 0:
                self.write_line_break()
            self.state = self.states.pop()
        else:
            self.write_indicator("", True)  # ADDED  <-----------------------------------------------
            if self.canonical or self.column > self.best_width:
                self.write_indent()
            self.states.append(self.expect_flow_sequence_item)
            self.expect_node(sequence=True)

    def expect_flow_sequence_item(self) -> None:
        if isinstance(self.event, SequenceEndEvent):
            self.indent = self.indents.pop()
            popped = self.flow_context.pop()
            assert popped == "["  # noqa
            if self.canonical:
                # ToDo: so-39595807, maybe add a space to the flow_seq_separator
                # and strip the last space, if space then indent, else do not
                # not sure that [1,2,3] is a valid YAML seq
                self.write_indicator(self.flow_seq_separator, False)
                self.write_indent()
            self.write_indicator(self.flow_seq_end, True)  # EDITED  <-----------------------------------------------
            if self.event.comment and self.event.comment[0]:
                # eol comment on flow sequence
                self.write_post_comment(self.event)
            else:
                self.no_newline = False
            self.state = self.states.pop()
        else:
            self.write_indicator(self.flow_seq_separator, False)
            if self.canonical or self.column > self.best_width:
                self.write_indent()
            self.states.append(self.expect_flow_sequence_item)
            self.expect_node(sequence=True)


ruamel= yaml.YAML(typ=typ)
ruamel.Emitter = Emitter

---

Modification proposal

As we can see, the correction to do is very small. I would like to add an option to control the number of spaces at the start/end of the sequence that would only be used here. I can make the PR but I want to confirm that someone will be checking, hopefully approving it, and that the names chosen will be validated.

Thank you