FIx Makefile with Python 3 command names

Use `python3` instead of `python`.

Try `pytest-3` if `pytest` is not available.

Makefile

@@ -29,6 +29,8 @@ check-dictionaries:
 	done
 	@if command -v pytest > /dev/null; then \
 		pytest codespell_lib/tests/test_dictionary.py; \
+	elif command -v pytest-3 > /dev/null; then \
+		pytest-3 codespell_lib/tests/test_dictionary.py; \
 	else \
 		echo "Test dependencies not present, install using 'pip install -e \".[dev]\"'"; \
 		exit 1; \
@@ -48,13 +50,20 @@ check-manifest:
 	check-manifest --no-build-isolation
 
 check-distutils:
-	python setup.py check --restructuredtext --strict
+	python3 setup.py check --restructuredtext --strict
 
 flake8:
 	flake8
 
 pytest:
-	pytest codespell_lib
+	@if command -v pytest > /dev/null; then \
+		pytest codespell_lib; \
+	elif command -v pytest-3 > /dev/null; then \
+		pytest-3 codespell_lib; \
+	else \
+		echo "Test dependencies not present, install using 'pip install -e \".[dev]\"'"; \
+		exit 1; \
+	fi
 
 clean:
 	rm -rf codespell.1