fix: parse sign_tags from config as boolean

[maimul]

Problem

Setting sign_tags = False in .bumpversion.cfg has no effect. The boolean-parsing loop in _load_configuration only covers commit, tag, and dry_run, so sign_tags is never read from the config file. Users who set sign_tags = False to suppress GPG signing still get signing attempts, which fail if no GPG key is configured.

Fix

Add sign_tags to the boolean-parsing loop (one-line change in cli.py):

# before
for boolvaluename in ("commit", "tag", "dry_run"):

# after
for boolvaluename in ("commit", "tag", "dry_run", "sign_tags"):

Reproducer

# .bumpversion.cfg
[bumpversion]
current_version = 1.0.0
sign_tags = False

Before this fix, sign_tags remains True (the argparse default), so bumpversion attempts GPG signing. After this fix it is correctly False.

Fixes #269