Parameterize python binary in Makefile

Although ideally, all python minor versions (3.x) would be backwards
compatible, unfortunately that is not the case.  Consequently, sometimes
developers will have multiple python builds available on their systems,
not just a single `python3`.  This change simply makes the python binary
name a parameter of the Makefile so that affected developers may pass:

```
PYTHON=python-3.9 make
```

Or similar.

Makefile

@@ -3,6 +3,9 @@ ITESTS_L2_HOST=http://localhost:9545
 BEDROCK_TAGS_REMOTE?=origin
 OP_STACK_GO_BUILDER?=us-docker.pkg.dev/oplabs-tools-artifacts/images/op-stack-go:latest
 
+# Requires at least Python v3.9; specify a minor version below if needed
+PYTHON?=python3
+
 build: build-go build-ts
 .PHONY: build
 
@@ -114,14 +117,14 @@ pre-devnet:
 devnet-up: pre-devnet
 	./ops/scripts/newer-file.sh .devnet/allocs-l1.json ./packages/contracts-bedrock \
 		|| make devnet-allocs
-	PYTHONPATH=./bedrock-devnet python3 ./bedrock-devnet/main.py --monorepo-dir=.
+	PYTHONPATH=./bedrock-devnet $(PYTHON) ./bedrock-devnet/main.py --monorepo-dir=.
 .PHONY: devnet-up
 
 # alias for devnet-up
 devnet-up-deploy: devnet-up
 
 devnet-test: pre-devnet
-	PYTHONPATH=./bedrock-devnet python3 ./bedrock-devnet/main.py --monorepo-dir=. --test
+	PYTHONPATH=./bedrock-devnet $(PYTHON) ./bedrock-devnet/main.py --monorepo-dir=. --test
 .PHONY: devnet-test
 
 devnet-down:
@@ -137,7 +140,7 @@ devnet-clean:
 .PHONY: devnet-clean
 
 devnet-allocs: pre-devnet
-	PYTHONPATH=./bedrock-devnet python3 ./bedrock-devnet/main.py --monorepo-dir=. --allocs
+	PYTHONPATH=./bedrock-devnet $(PYTHON) ./bedrock-devnet/main.py --monorepo-dir=. --allocs
 
 devnet-logs:
 	@(cd ./ops-bedrock && docker compose logs -f)