package/python-setuptools: add comment about python3-setuptools

As suggested by Arnout, add a comment to indicate that the version of
python-setuptools must be kept in sync with python3-setuptools.

Suggested-by: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/python-setuptools/python-setuptools.mk

@@ -4,6 +4,8 @@
 #
 ################################################################################
 
+# Please keep in sync with
+# package/python3-setuptools/python3-setuptools.mk
 PYTHON_SETUPTOOLS_VERSION = 40.6.3
 PYTHON_SETUPTOOLS_SOURCE = setuptools-$(PYTHON_SETUPTOOLS_VERSION).zip
 PYTHON_SETUPTOOLS_SITE = https://files.pythonhosted.org/packages/37/1b/b25507861991beeade31473868463dad0e58b1978c209de27384ae541b0b