🐛 Fix the need of httptools on minimal installation

[Kludex]

This solves the following bug, caused when you installed uvicorn via pip install uvicorn.

>>> import uvicorn
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/marcelo/Development/encode/uvicorn/uvicorn/__init__.py", line 2, in <module>
    from uvicorn.main import Server, main, run
  File "/home/marcelo/Development/encode/uvicorn/uvicorn/main.py", line 23, in <module>
    from uvicorn.server import Server, ServerState  # noqa: F401  # Used to be defined here.
  File "/home/marcelo/Development/encode/uvicorn/uvicorn/server.py", line 19, in <module>
    from uvicorn.protocols.http.httptools_impl import HttpToolsProtocol
  File "/home/marcelo/Development/encode/uvicorn/uvicorn/protocols/http/httptools_impl.py", line 8, in <module>
    import httptools
ModuleNotFoundError: No module named 'httptools'

[euri10]

lgtm

[Kludex]

Thanks!