Add a message if bin/qmk doesn't work (qmk#9000)

Makefile

@@ -562,8 +562,8 @@ endef
 %:
 	# Check if we have the CMP tool installed
 	cmp $(ROOT_DIR)/Makefile $(ROOT_DIR)/Makefile >/dev/null 2>&1; if [ $$? -gt 0 ]; then printf "$(MSG_NO_CMP)"; exit 1; fi;
-	# Ensure that python3 is installed. This check can be removed after python is used in more places.
-	if ! python3 --version 1> /dev/null 2>&1; then printf "$(MSG_PYTHON_MISSING)"; fi
+	# Ensure that bin/qmk works. This will be a failing check after the next develop merge on 2020 Aug 29.
+	if ! bin/qmk hello 1> /dev/null 2>&1; then printf "$(MSG_PYTHON_MISSING)"; fi
 	# Check if the submodules are dirty, and display a warning if they are
 ifndef SKIP_GIT
 	if [ ! -e lib/chibios ]; then git submodule sync lib/chibios && git submodule update --depth 50 --init lib/chibios; fi

docs/ChangeLog/20200530/PR9000.md

@@ -0,0 +1,5 @@
+# Python Required In The Build Process
+
+This is the last release of QMK that will work without having Python 3.6 (or later) installed. If your environment is not fully setup you will get a warning instructing you to set it up.
+
+After the next breaking change you will not be able to build if `bin/qmk hello` does not work.

message.mk

@@ -84,8 +84,7 @@ MSG_FILE_TOO_SMALL = The firmware is too small! $(CURRENT_SIZE)/$(MAX_SIZE)\n
 MSG_FILE_JUST_RIGHT = The firmware size is fine - $(CURRENT_SIZE)/$(MAX_SIZE) ($(PERCENT_SIZE)%%, $(FREE_SIZE) bytes free)\n
 MSG_FILE_NEAR_LIMIT = The firmware size is approaching the maximum - $(CURRENT_SIZE)/$(MAX_SIZE) ($(PERCENT_SIZE)%%, $(FREE_SIZE) bytes free)\n
 MSG_PYTHON_MISSING = $(WARN_COLOR)WARNING:$(NO_COLOR)\n \
-	Python 3 is not installed. It will be required by a future version\n\
-	of qmk_firmware.\n\n\
+	Can not run bin/qmk! This tool will be required when the develop branch is merged on 2020 Aug 29.\n\n\
 	Please run $(BOLD)util/qmk_install.sh$(NO_COLOR) to install all the dependencies QMK requires.\n\n
 MSG_FLASH_BOOTLOADER = $(WARN_COLOR)WARNING:$(NO_COLOR) This board's bootloader is not specified or is not supported by the \":flash\" target at this time.\n\n
 MSG_FLASH_ARCH = $(WARN_COLOR)WARNING:$(NO_COLOR) This board's architecture is not supported by the \":flash\" target at this time.\n\n