Mark 'sign_tags' as a boolean value

[julien-lecomte]

When passed as a command line option, --(no-)sign-tags is a boolean. When only taken from the configuration, it's a string.

Because of this, string "false" later evaluate to true and signing is attempted and fails.

Working configuration:

[bumpversion]
current_version = 0.2.8
commit = true
tag = true
message = Bump version from {current_version} to {new_version}
tag_message = Release version {new_version} (previous: {current_version})

[bumpversion:file:pyproject.toml]

Broken configuration:

[bumpversion]
current_version = 0.2.8
commit = true
tag = true
sign_tags = false
message = Bump version from {current_version} to {new_version}
tag_message = Release version {new_version} (previous: {current_version})

[bumpversion:file:pyproject.toml]