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>

package/python-markdown/Config.in

@@ -1,8 +1,8 @@
 config BR2_PACKAGE_PYTHON_MARKDOWN
 	bool "python-markdown"
+	depends on BR2_PACKAGE_PYTHON3
 	# runtime dependency on Python's xml module
-	select BR2_PACKAGE_PYTHON_PYEXPAT if BR2_PACKAGE_PYTHON
-	select BR2_PACKAGE_PYTHON3_PYEXPAT if BR2_PACKAGE_PYTHON3
+	select BR2_PACKAGE_PYTHON3_PYEXPAT
 	help
 	  This is a Python implementation of John Gruber's Markdown