Makefile: Use python3 by default

Makefile

@@ -1,3 +1,5 @@
+PYTHON := python3
+PYTEST := pytest
 PYTEST_ARGS := -W ignore::DeprecationWarning -vv -x --log-level=DEBUG
 JULIA := julia --color=yes --project=src/julia --sysimage build/sysimage.so
 
@@ -8,14 +10,15 @@ build/sysimage.so: src/julia/Manifest.toml src/julia/Project.toml
 	julia --color=yes --project=src/julia src/julia/sysimage.jl
 
 develop:
-	cd src/python && python setup.py develop
+	cd src/python && $(PYTHON) setup.py develop
-	$(JULIA) -e "using Pkg; Pkg.instantiate()"
 
 docs:
 	mkdocs build
 
 install:
-	cd src/python && python setup.py install
+	cd src/python && $(PYTHON) setup.py install
+
+install-julia:
 	$(JULIA) -e "using Pkg; Pkg.instantiate()"
 
 uninstall:
@@ -24,7 +27,7 @@ uninstall:
 test: test-python test-julia
 
 test-python:
-	cd src/python && pytest $(PYTEST_ARGS)
+	cd src/python && $(PYTEST) $(PYTEST_ARGS)
 
 test-python-watch:
 	cd src/python && pytest-watch -- $(PYTEST_ARGS)