Add typing_extensions dep.

[Icantjuddle]

Currently, when running on older versions of python before 3.8, Typer requires the use of typing_extensions for the Literal type.

https://github.com/tiangolo/typer/blob/6208b5b2087d3474023cdf650a246c9a6c8327c3/typer/core.py#L30-L33

This was added to that library after typing_extensions 3.6.6 release (diff here).

When I run on python3.7 I'm seeing issues when testing my typer clis due to this missing dep.

I'm not the most familiar with the python packaging ecosystem but I pulled the syntax from here:
https://flit.pypa.io/en/stable/pyproject_toml.html#dependencies

[github-actions]

📝 Docs preview for commit 7320fc5 at: https://63767105747f9411682c20d3--typertiangolo.netlify.app

[github-actions]

📝 Docs preview for commit b4b07eb at: https://639ce9d92c118d0a1647f762--typertiangolo.netlify.app

[Icantjuddle]

@tiangolo Let me know if there is anything more I can add/test/show here to help.

[Icantjuddle]

@tiangolo adding a repro to demonstrate the issue, would be great if we could merge. Let me know if there is anything else I can do to help, test, etc.

❯ python -m venv ./test_env
❯ source ./test_env/bin/activate
❯ python --version
Python 3.7.9
❯ pip install click==7.1.2
Collecting click==7.1.2
  Using cached click-7.1.2-py2.py3-none-any.whl (82 kB)
Installing collected packages: click
Successfully installed click-7.1.2
WARNING: You are using pip version 20.1.1; however, version 22.3.1 is available.
You should consider upgrading via the '/home/bjudd/code/typer_example/test_env/bin/python -m pip install --upgrade pip' command.
❯ pip install typer
Collecting typer
  Using cached typer-0.7.0-py3-none-any.whl (38 kB)
Requirement already satisfied: click<9.0.0,>=7.1.1 in ./test_env/lib/python3.7/site-packages (from typer) (7.1.2)
Installing collected packages: typer
Successfully installed typer-0.7.0
WARNING: You are using pip version 20.1.1; however, version 22.3.1 is available.
You should consider upgrading via the '/home/bjudd/code/typer_example/test_env/bin/python -m pip install --upgrade pip' command.
❯ python -m typer
Traceback (most recent call last):
  File "/home/bjudd/.local/apps/pyenv/versions/3.7.9/lib/python3.7/runpy.py", line 183, in _run_module_as_main
    mod_name, mod_spec, code = _get_module_details(mod_name, _Error)
  File "/home/bjudd/.local/apps/pyenv/versions/3.7.9/lib/python3.7/runpy.py", line 142, in _get_module_details
    return _get_module_details(pkg_main_name, error)
  File "/home/bjudd/.local/apps/pyenv/versions/3.7.9/lib/python3.7/runpy.py", line 109, in _get_module_details
    __import__(pkg_name)
  File "/home/bjudd/code/typer_example/test_env/lib/python3.7/site-packages/typer/__init__.py", line 30, in <module>
    from .main import Typer as Typer
  File "/home/bjudd/code/typer_example/test_env/lib/python3.7/site-packages/typer/main.py", line 17, in <module>
    from .core import MarkupMode, TyperArgument, TyperCommand, TyperGroup, TyperOption
  File "/home/bjudd/code/typer_example/test_env/lib/python3.7/site-packages/typer/core.py", line 33, in <module>
    from typing_extensions import Literal
ModuleNotFoundError: No module named 'typing_extensions'
❯ pip install pipdeptree
Collecting pipdeptree
  Using cached pipdeptree-2.3.3-py3-none-any.whl (16 kB)
Installing collected packages: pipdeptree
Successfully installed pipdeptree-2.3.3
WARNING: You are using pip version 20.1.1; however, version 22.3.1 is available.
You should consider upgrading via the '/home/bjudd/code/typer_example/test_env/bin/python -m pip install --upgrade pip' command.
❯ pipdeptree
pip==20.1.1
pipdeptree==2.3.3
setuptools==47.1.0
typer==0.7.0
  - click [required: >=7.1.1,<9.0.0, installed: 7.1.2]

If you update the version of click to something newer, click itself a transitive dependency on typing-extensions so typer can get away with not declaring its own, but for older versions of click this does not work and the dependency is missing.

[ryangalamb]

This change seems reasonable. If #584 is merged in its current state, we can close this. (#584 currently adds typing_extensions to the deps.)

If #584 doesn't get merged, or it gets changed not to include typing_extensions, I'll take another look at this one.

[Icantjuddle]

Thanks @ryangalamb, looks like your PR got merged so this can be closed.