Changed default Python3 version for venv to 3.7.

(Redmine issue: #3443)

Makefile

@@ -35,7 +35,7 @@ DIR_EVENT_CLASSES = $(shell pwd)/conf/event_classes
 BIN_FILES := $(wildcard bin/mentat-*.py)
 LIB_FILES := $(shell find $(DIR_LIB) -name '*.py' | grep -v 'test_')
 
-VENV_PYTHON ?= python3
+VENV_PYTHON ?= python3.7
 VENV_PATH   ?= venv
 
 PYTHON      = python3
@@ -94,7 +94,7 @@ help:
 	@echo "  * $(GREEN)help$(NC): print this help message and exit"
 	@echo "  * $(GREEN)show-version$(NC): show current project version"
 	@echo "  * $(GREEN)show-envstamp$(NC): show information about current development environment"
-	@echo "  * $(GREEN)venv [VENV_PYTHON=python3] [VENV_PATH=venv]$(NC): install Python virtual environment for local development"
+	@echo "  * $(GREEN)venv [VENV_PYTHON=python3.7] [VENV_PATH=venv]$(NC): install Python virtual environment for local development"
 	@echo "  * $(GREEN)develop$(NC): install and configure project locally for development"
 	@echo "  * $(GREEN)deps$(NC): install project dependencies"
 	@echo "  * $(GREEN)clean$(NC): cleanup development and build environment"