package/python-markdown: fix Python version dependency

package/python-markdown: fix Python version dependency

Since upstream commit
https://github.com/Python-Markdown/markdown/commit/dab931f597cdc5b57d4e2458840fb991606506bc,
merged since version 3.2 of python-markdown, the Python 2.x support
has been dropped.

So our commit 6e538bf642d3584032d63bb54d24b88ab57e4283
("package/python-markdown: bump to version 3.2.1") should have made
python-markdown available only for Python 3.x.

Fixes:
http://autobuild.buildroot.net/results/6d1e796f6658accb3eafef265935db5e464638fe

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Reviewed-by: Asaf Kahlon <asafka7@gmail.com>
Tested-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>