textproc/py-sphinx-markdown-tables: return to 0.0.15

This port was prematurely upgraded to 0.0.17 which required py-markdown
3.4.0 or later while the ports system has 3.3.7. This broke builds of
llvm13 and higher. Upgrading py-markdown appears to require significant
testing and coordination.

This reverts commit 5a0fd27b2f9b47b1208000e8f26a735885bee196.

PR: 265839
Approved by: wen