Makefile: fixes for working on debian based system

Setup up PY variable because the `python` binary isn't found
in these systems. Instead there is only `python3`.

So, you can run there commands with:

$ make foo PY=python3

Makefile

@@ -1,3 +1,4 @@
+SHELL := bash
 get_venv_dir:=$(shell mktemp -d 2>/dev/null || mktemp -d -t 'tmpvenv')
 venv_dir := $(get_venv_dir)/pipenv_venv
 venv_file := $(CURDIR)/.test_venv
@@ -18,6 +19,7 @@ write_git_tmpdir = $(file > $(GITDIR_STAMPFILE),$(create_git_tmpdir))
 get_checkout_dir = $(file < $(GITDIR_STAMPFILE))
 get_checkout_subdir = $(addprefix $(get_checkout_dir), $(1))
 pip-checkout-dir = $(get_checkout_dir)/patch-pip
+PY ?= python
 
 format:
 	black pipenv/*.py
@@ -47,7 +49,7 @@ ramdisk:
 .PHONY: ramdisk-virtualenv
 ramdisk-virtualenv: ramdisk
 	[ ! -e "/mnt/ramdisk/.venv/bin/activate" ] && \
-		python -m venv /mnt/ramdisk/.venv
+		$(PY) -m venv /mnt/ramdisk/.venv
 	echo "/mnt/ramdisk/.venv" > $(venv_file)
 
 .PHONY: virtualenv