FIx Makefile with Python 3 command names

Use `python3` instead of `python`. Command `python` is not available
by default on recent distributions such as Ubuntu 22.04.

Command `pytest` is not available on distributions such as Ubuntu 22.04,
only `pytest-3` is available. Work around by running `python -m pytest`
instead of `pytest` as suggested by README.Debian:
https://salsa.debian.org/python-team/packages/pytest/-/blob/master/debian/README.Debian

Makefile

@@ -27,12 +27,7 @@ check-dictionaries:
 			exit 1; \
 		fi; \
 	done
-	@if command -v pytest > /dev/null; then \
-		pytest codespell_lib/tests/test_dictionary.py; \
-	else \
-		echo "Test dependencies not present, install using 'pip install -e \".[dev]\"'"; \
-		exit 1; \
-	fi
+	python3 -m pytest codespell_lib/tests/test_dictionary.py
 
 sort-dictionaries:
 	@for dictionary in ${DICTIONARIES}; do \
@@ -48,13 +43,13 @@ 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
+	python3 -m pytest codespell_lib/tests/test_dictionary.py
 
 clean:
 	rm -rf codespell.1