A HTTP Request to crates.io with Go causes 403 (violation of crates.io crawler policy)

main.go

package main

import (
	"fmt"
	"io"
	"log"
	"net/http"
	"os"
)

func main() {
	if err := core(); err != nil {
		log.Fatal(err)
	}
}

func core() error {
	// curl -o /dev/null -w '%{http_code}\n' -s https://crates.io/api/v1/crates/skim/versions
	// 200
	req, err := http.NewRequest("GET", "https://crates.io/api/v1/crates/skim/versions", nil)
	if err != nil {
		return err
	}
	resp, err := http.DefaultClient.Do(req)
	if err != nil {
		return err
	}
	defer resp.Body.Close()
	fmt.Println(resp.StatusCode) // 403
	// {"errors":[{"detail":"We are unable to process your request at this time. This usually means that you are in violation of our crawler policy (https://crates.io/policies#crawlers). Please open an issue at https://github.com/rust-lang/crates.io or email [email protected] and provide the request id xxx"}]}
	if _, err := io.Copy(os.Stdout, resp.Body); err != nil {
		return err
	}
	return nil
}