chore: use $(...) notation instead of legacy backticked ... in Makefile (#344)

Author: jenstroeger

Makefile

@@ -103,7 +103,7 @@ force-upgrade:
 	rm -f .venv/upgraded-on
 	$(MAKE) upgrade
 upgrade-quiet:
-	echo "Automatically generated by Python Package Makefile on `date '+%Y-%m-%d %H:%M:%S %z'`." > .venv/upgraded-on
+	echo "Automatically generated by Python Package Makefile on $$(date '+%Y-%m-%d %H:%M:%S %z')." > .venv/upgraded-on
 
 # Generate a Software Bill of Materials (SBOM).
 .PHONY: sbom
@@ -123,18 +123,18 @@ sbom: requirements
 requirements: requirements.txt
 requirements.txt: pyproject.toml
 	echo -n "" > requirements.txt
-	for pkg in `python -m pip freeze --local --disable-pip-version-check --exclude-editable`; do \
+	for pkg in $$(python -m pip freeze --local --disable-pip-version-check --exclude-editable); do \
 	  pkg=$${pkg//[$$'\r\n']}; \
 	  echo -n $$pkg >> requirements.txt; \
 	  echo "Fetching package metadata for requirement '$$pkg'"; \
 	  [[ $$pkg =~ (.*)==(.*) ]] && curl -s https://pypi.org/pypi/$${BASH_REMATCH[1]}/$${BASH_REMATCH[2]}/json | python -c "import json, sys; print(''.join(f''' \\\\\n    --hash=sha256:{pkg['digests']['sha256']}''' for pkg in json.load(sys.stdin)['urls']));" >> requirements.txt; \
 	done
 	echo -e -n "$(PACKAGE_NAME)==$(PACKAGE_VERSION)" >> requirements.txt
 	if [ -f dist/$(PACKAGE_NAME)-$(PACKAGE_VERSION).tar.gz ]; then \
-	  echo -e -n " \\\\\n    `python -m pip hash --algorithm sha256 dist/$(PACKAGE_NAME)-$(PACKAGE_VERSION).tar.gz | grep '^\-\-hash'`" >> requirements.txt; \
+	  echo -e -n " \\\\\n    $$(python -m pip hash --algorithm sha256 dist/$(PACKAGE_NAME)-$(PACKAGE_VERSION).tar.gz | grep '^\-\-hash')" >> requirements.txt; \
 	fi
 	if [ -f dist/$(PACKAGE_NAME)-$(PACKAGE_VERSION)-py3-none-any.whl ]; then \
-	  echo -e -n " \\\\\n    `python -m pip hash --algorithm sha256 dist/$(PACKAGE_NAME)-$(PACKAGE_VERSION)-py3-none-any.whl | grep '^\-\-hash'`" >> requirements.txt; \
+	  echo -e -n " \\\\\n    $$(python -m pip hash --algorithm sha256 dist/$(PACKAGE_NAME)-$(PACKAGE_VERSION)-py3-none-any.whl | grep '^\-\-hash')" >> requirements.txt; \
 	fi
 	echo "" >> requirements.txt
 	cp requirements.txt dist/$(PACKAGE_NAME)-$(PACKAGE_VERSION)-requirements.txt
@@ -145,7 +145,7 @@ requirements.txt: pyproject.toml
 # found an issue.
 .PHONY: audit
 audit:
-	if ! `python -c "import pip_audit" &> /dev/null`; then \
+	if ! $$(python -c "import pip_audit" &> /dev/null); then \
 	  echo "No package pip_audit installed, upgrade your environment!" && exit 1; \
 	fi;
 	python -m pip_audit --skip-editable --desc on --fix --dry-run