py-strpdatetime: update to 0.4.0, add Python 3.13 subport

[breun]

Description

Update to 0.4.0, add Python 3.13 subport.

Tested on

macOS 15.1.1 24B91 arm64
Xcode 16.1 16B40

Verification

Have you

• followed our Commit Message Guidelines?
• squashed and minimized your commits?
• checked that there aren't other open pull requests for the same change?
• checked your Portfile with port lint?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?
• checked that the Portfile's most important variants haven't been broken?

[macportsbot]

Notifying maintainers:
@RhetTbull for port py-strpdatetime.

[reneeotten]

dependencies should be added in the same PR (cf. #26959) as otherwise the CI will never pass. The py38 subport here and for py-textx should be removed; there are no other dependents in MacPorts and Python 3.8 is EOL.