修复 windows 平台 sea new 会报错的问题

[LKI]

原报错信息：

Traceback (most recent call last):
  File "C:\Users\liriansu\.virtualenvs\poseidon-tt7N4OpO\Scripts\sea-script.py", line 11, in <module>
    load_entry_point('sea==2.2.0', 'console_scripts', 'sea')()
  File "c:\users\liriansu\.virtualenvs\poseidon-tt7n4opo\lib\site-packages\sea-2.2.0-py3.8.egg\sea\cli.py", line 113, in main
    return _run(root)
  File "c:\users\liriansu\.virtualenvs\poseidon-tt7n4opo\lib\site-packages\sea-2.2.0-py3.8.egg\sea\cli.py", line 100, in _run
    return handler(**kwargs)
  File "c:\users\liriansu\.virtualenvs\poseidon-tt7n4opo\lib\site-packages\sea-2.2.0-py3.8.egg\sea\cmds.py", line 129, in new
    _gen_project(path, skip=_build_skip_files(extra), ctx=ctx)
  File "c:\users\liriansu\.virtualenvs\poseidon-tt7n4opo\lib\site-packages\sea-2.2.0-py3.8.egg\sea\cmds.py", line 117, in _gen_project
    tmpl = env.get_template(relfn)
  File "c:\users\liriansu\.virtualenvs\poseidon-tt7n4opo\lib\site-packages\jinja2\environment.py", line 883, in get_template
    return self._load_template(name, self.make_globals(globals))
  File "c:\users\liriansu\.virtualenvs\poseidon-tt7n4opo\lib\site-packages\jinja2\environment.py", line 857, in _load_template
    template = self.loader.load(self, name, globals)
  File "c:\users\liriansu\.virtualenvs\poseidon-tt7n4opo\lib\site-packages\jinja2\loaders.py", line 115, in load
    source, filename, uptodate = self.get_source(environment, name)
  File "c:\users\liriansu\.virtualenvs\poseidon-tt7n4opo\lib\site-packages\jinja2\loaders.py", line 177, in get_source
    pieces = split_template_path(template)
  File "c:\users\liriansu\.virtualenvs\poseidon-tt7n4opo\lib\site-packages\jinja2\loaders.py", line 32, in split_template_path
    raise TemplateNotFound(template)
jinja2.exceptions.TemplateNotFound: app\async_tasks.py.tmpl

因为在 jinja2/loaders.py 中使用的是 / 分隔符，windows 上用的是 \ 分隔符。
而 jinja 的哲学是统一都 replace 为 /……（这个风格不一定对，但他的项目 api 都这样）

所以我们调用的时候先修一下 :)

closes #132

[coveralls]

Coverage increased (+0.007%) to 93.81% when pulling 2607827 on LKI:fix/windows-jinja into b906ef9 on shanbay:master.

[LKI]

😉 我开一个 fork 分支支持一下 windows