Add Label functionality

[luanmingyi]

• Add methods for creating and retrieving labels in the Organization and Repository classes
• Add methods for retrieving and setting labels in the Issue class
• Introduce a new Label class to represent label objects
• Add some parsing information to Issue
• Update related imports and API object lists

tested with api 1.21.0

test code:

from gitea import (
    Gitea,
    User,
    Organization,
    Team,
    Repository,
    Branch,
    NotFoundException,
    AlreadyExistsException,
    Issue,
    Milestone,
    Label,
)

def generate_random_color():
    import random
    return "{:06x}".format(random.randint(0, 0xFFFFFF))

def print_label(l):
    print(l.id, l.name, l.color, l.exclusive, l.is_archived)
    print(l.description)
    print(l.url)

gitea = Gitea('https://try.gitea.cn','TOKEN')

print(gitea.get_version())

org = Organization.request(gitea, 'testorg')
l=org.create_label('org-new-label-1', generate_random_color())
l=org.create_label('org-new-label-2', generate_random_color())
l=org.create_label('org-new-label-3', generate_random_color())

repo = Repository.request(gitea, 'testorg', 'testpy')
l=repo.create_label('repo-new-label-1', generate_random_color())
l=repo.create_label('repo-new-label-2', generate_random_color())
l=repo.create_label('repo-new-label-3', generate_random_color())

l = Label.request(gitea, 'testorg', None, 1)
print_label(l)
l = Label.request(gitea, 'testorg', 'testpy', 4)
print_label(l)

for l in org.get_labels():
    print('---o ', l.name)
    l.color = generate_random_color()
    l.commit()

for l in repo.get_labels():
    print('---r ', l.name)
    l.color = generate_random_color()
    l.commit()

issue:Issue = Issue.request(gitea, 'testorg', 'testpy', 1)
for l in issue.labels:
    print('---i ', l.name)

issue.set_labels([org.get_labels()[1],repo.get_labels()[2]])
issue.set_labels([])

print(issue.due_date)
from datetime import datetime,timedelta
issue.due_date = issue.created_at+timedelta(days=5)
issue.commit()