Drop isErrClosed

We require a new enough Go version now.

conn.go

@@ -2,6 +2,7 @@ package soju
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"io"
 	"net"
@@ -163,7 +164,7 @@ func newConn(srv *Server, ic ircConn, options *connOptions) *conn {
 				break
 			}
 		}
-		if err := c.conn.Close(); err != nil && !isErrClosed(err) {
+		if err := c.conn.Close(); err != nil && !errors.Is(err, net.ErrClosed) {
 			c.logger.Printf("failed to close connection: %v", err)
 		} else {
 			c.logger.Debugf("connection closed")
@@ -202,7 +203,7 @@ func (c *conn) Close() error {
 
 func (c *conn) ReadMessage() (*irc.Message, error) {
 	msg, err := c.conn.ReadMessage()
-	if isErrClosed(err) {
+	if errors.Is(err, net.ErrClosed) {
 		return nil, io.EOF
 	} else if err != nil {
 		return nil, err

net_go113.go

@@ -1,12 +0,0 @@
-//go:build !go1.16
-// +build !go1.16
-
-package soju
-
-import (
-	"strings"
-)
-
-func isErrClosed(err error) bool {
-	return err != nil && strings.Contains(err.Error(), "use of closed network connection")
-}

net_go116.go

@@ -1,13 +0,0 @@
-//go:build go1.16
-// +build go1.16
-
-package soju
-
-import (
-	"errors"
-	"net"
-)
-
-func isErrClosed(err error) bool {
-	return errors.Is(err, net.ErrClosed)
-}

server.go

@@ -581,7 +581,7 @@ func (s *Server) Serve(ln net.Listener, handler func(ircConn)) error {
 
 	for {
 		conn, err := ln.Accept()
-		if isErrClosed(err) {
+		if errors.Is(err, net.ErrClosed) {
 			return nil
 		} else if err != nil {
 			return fmt.Errorf("failed to accept connection: %v", err)