Description of proposed changes

Changes in this PR:

1. -U's +t modifier was added Let -U take +ttext to override time stamp gmt#7127, so the text parameter doesn't work for GMT<=6.4.0. This PR implements the same feature for GMT<=6.4.0 by overrriding the timefmt parameter that will be passed to FORMAT_TIME_STAMP. Now, the text parameter works the same for any GMT versions.
2. Text string passed to the text parameter will be truncated to 64 characters if its length exceeds 64. Now Figure.timestamp() raise a warning instead of an exception when getting a too long text argument.
3. Add a test for passing a very long text string to the text parameter.

Fixes #

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.
• Use underscores (not hyphens) in names of Python files and directories.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash commands are:

• /format: automatically format and lint the code
• /test-gmt-dev: run full tests on the latest GMT development version