go api_* files fail to import time module

Description

go client api_* files do not import time module when the generated code contains references to time.Time.

This results in go compilation failure

Swagger-codegen version

3.0.20

Swagger declaration file content or url

dt_test1.yml

openapi: 3.0.0
info:
  title: DateTime DataObject Test
  version: version.version0

paths:
  /test_fail:
    post:
      operationId: DateTimeImportFail
      responses:
        '200':
          content:
            "application/json":
              schema:
                type: string
                format: date-time

Command line used for generation

java -jar swagger-codegen-cli.jar generate -l go -i dt_test1.yml -o test/go -c swagger.json

$ cat swagger.json 
{
  "packageName": "main"
}

Steps to reproduce

Define an operaiton returning date-time string value and generate go bindings. The results will refer time.Time type and not import the time module

package main

import (
	"context"
	"io/ioutil"
	"net/http"
	"net/url"
	"strings"
)
.......
func (a *DefaultApiService) DateTimeImportFail(ctx context.Context) (time.Time, *http.Response, error) {

This does not reproduce for data objects/models containing date time values

Related issues/PRs

Cannot find any

Suggest a fix/enhancement

import go time module when used in api_*.go files

Start with the generated api_*.go import block and reproduce the failure using dt_test1.yml, swagger.json, and the documented swagger-codegen generate command. Done means the generated DateTimeImportFail method's time.Time reference has its required import and the generated Go client compiles.