Build: Make Makefile use python3 instead of whatever "python" is.

Author: scoder

Makefile

@@ -1,7 +1,7 @@
-PYTHON?=python
+PYTHON?=python3
 VERSION?=$(shell sed -ne "s|^__version__\s*=\s*'\([^']*\)'.*|\1|p" src/quicktions.pyx)
 PACKAGE=quicktions
-WITH_CYTHON := $(shell python -c 'from Cython.Build import cythonize' 2>/dev/null && echo "--with-cython")
+WITH_CYTHON := $(shell $(PYTHON) -c 'from Cython.Build import cythonize' 2>/dev/null && echo "--with-cython")
 PYTHON_WHEEL_BUILD_VERSION := "cp*"
 
 MANYLINUX_IMAGES= \