remove uses of errors.Is, which requires go1.13

Commit 1bf832c introduced the use of
`errors.Is`, which was added in [go1.13], but the `go.mod` was not
updated to reflect this, causing compile failures on go1.12:

    docker run -it --rm -v ./:/pflag -w /pflag golang:1.12 sh -c 'go test -v ./...'
    # github.com/spf13/pflag [github.com/spf13/pflag.test]
    ./flag.go:1190:7: undefined: errors.Is
    ./flag.go:1219:7: undefined: errors.Is
    ./bool_func_test.go:86:28: cannot use stdFSet (type *flag.FlagSet) as type BoolFuncFlagSet in argument to runCase:
        *flag.FlagSet does not implement BoolFuncFlagSet (missing BoolFunc method)
    ...

As the error that is tested will not be wrapped, we can omit the `errors.Is`,
and instead continue doing a straight comparison.

[go1.13]: https://pkg.go.dev/errors@go1.13#Is

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Author: thaJeztah

flag.go

@@ -1185,7 +1185,7 @@ func (f *FlagSet) Parse(arguments []string) error {
 		case ContinueOnError:
 			return err
 		case ExitOnError:
-			if errors.Is(err, ErrHelp) {
+			if err == ErrHelp {
 				os.Exit(0)
 			}
 			fmt.Fprintln(f.Output(), err)
@@ -1214,7 +1214,7 @@ func (f *FlagSet) ParseAll(arguments []string, fn func(flag *Flag, value string)
 		case ContinueOnError:
 			return err
 		case ExitOnError:
-			if errors.Is(err, ErrHelp) {
+			if err == ErrHelp {
 				os.Exit(0)
 			}
 			fmt.Fprintln(f.Output(), err)