Upgrade protobuf dependency to 1.36.1, remove retool

CONTRIBUTING.md

@@ -34,8 +34,7 @@ Twirp uses github pull requests. Fork, hack away at your changes and submit. Mos
 
 - Add tests that cover your contribution. Overall code coverage should not decrease.
 - Twirp officially supports the last 3 releases of Go.
-- Protobuf version 3.x.x to generate code with the protoc command.
-- For linters and other tools, we use [retool](https://github.com/twitchtv/retool). If `make setup` is not able to install it, you can install it in your path with `go get github.com/twitchtv/retool` and then install tools with `retool build`.
+- Protobuf version 3.x.x or later to generate code with the protoc command.
 
 ### Running tests
 

Gopkg.toml

@@ -9,7 +9,7 @@
 
 [[override]]
   name = "google.golang.org/protobuf"
-  version = "1.26.0"
+  version = "1.36.1"
 
 [[override]]
   name = "github.com/pkg/errors"
@@ -23,3 +23,9 @@
   unused-packages = true
   go-tests = true
   non-go = true
+
+  [[prune.project]]
+    # Protobuf package uses go:embed to include files, so we need to avoid
+    # pruning them.
+    name = "google.golang.org/protobuf"
+    non-go = false

Makefile

@@ -1,4 +1,4 @@
-PATH := ${PWD}/_tools/bin:${PWD}/bin:${PATH}
+PATH := ${PWD}/bin:${PATH}
 export GO111MODULE=off
 
 all: setup test_all
@@ -7,8 +7,9 @@ all: setup test_all
 
 setup:
 	./check_protoc_version.sh
-	GOPATH="$$PWD/_tools" GOBIN="$$PWD/_tools/bin" go get github.com/twitchtv/retool
-	./_tools/bin/retool build
+	# Install dependent tools via modules
+	GO111MODULE=on GOBIN="$$PWD/bin" go install -v google.golang.org/protobuf/cmd/[email protected]
+	GO111MODULE=on GOBIN="$$PWD/bin" go install -v github.com/kisielk/[email protected]
 
 generate:
 	# Recompile and install generator
@@ -19,7 +20,7 @@ generate:
 test_all: setup test test_clientcompat
 
 test: generate
-	./_tools/bin/errcheck ./internal/twirptest
+	./bin/errcheck ./internal/twirptest
 	go test -race ./...
 
 test_clientcompat: generate