Allow to override Python binary for make (#887)

With this commit we expose a new make variable `PY_BIN` which defaults
to `python3` to keep the current behavior. However, this change allows
to experiment with other Python implementations like pypy by specifying
`make venv-create PY_BIN=pypy3` (provided pypy has been installed
correctly on the system; adding support for this is intentionally out of
scope for this PR). While this does not mean we will start supporting
pypy, it allows us to experiment with it.

Makefile

@@ -19,10 +19,11 @@ SHELL = /bin/bash
 # We assume an active virtualenv for development
 include make-requirements.txt
 PYENV_REGEX = .pyenv/shims
+PY_BIN = python3
 VENV_NAME ?= .venv
 VENV_ACTIVATE_FILE = $(VENV_NAME)/bin/activate
 VENV_ACTIVATE = . $(VENV_ACTIVATE_FILE)
-VEPYTHON = $(VENV_NAME)/bin/python3
+VEPYTHON = $(VENV_NAME)/bin/$(PY_BIN)
 VEPYLINT = $(VENV_NAME)/bin/pylint
 PYENV_ERROR = "\033[0;31mIMPORTANT\033[0m: Please install pyenv.\n"
 PYENV_PATH_ERROR = "\033[0;31mIMPORTANT\033[0m: Please add $(HOME)/$(PYENV_REGEX) to your PATH env.\n"
@@ -49,7 +50,7 @@ venv-create:
 		exit 1; \
 	fi;
 	@if [[ ! -f $(VENV_ACTIVATE_FILE) ]]; then \
-		eval "$$(pyenv init -)" && python3 -mvenv $(VENV_NAME); \
+		eval "$$(pyenv init -)" && $(PY_BIN) -mvenv $(VENV_NAME); \
 		printf "Created python3 venv under $(PWD)/$(VENV_NAME).\n"; \
 	fi;