FIx Makefile with Python 3 command names

Use `python3` instead of `python`. Command `python` is not available
by default on 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

Author: DimitriPapadopoulos

Makefile

@@ -27,8 +27,8 @@ check-dictionaries:
 			exit 1; \
 		fi; \
 	done
-	@if command -v pytest > /dev/null; then \
-		pytest codespell_lib/tests/test_dictionary.py; \
+	@if python3 -c 'import pytest'; then \
+		python3 -m pytest codespell_lib/tests/test_dictionary.py; \
 	else \
 		echo "Test dependencies not present, install using 'pip install -e \".[dev]\"'"; \
 		exit 1; \
@@ -48,13 +48,18 @@ 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 python3 -c 'import pytest'; then \
+		python3 -m pytest codespell_lib;	\
+	else \
+		echo "Test dependencies not present, install using 'pip install -e \".[dev]\"'"; \
+		exit 1; \
+	fi
 
 clean:
 	rm -rf codespell.1