Remove pinning of httptools==0.2.* - this blocks wheel usage of httptools on Python 3.10 for uvicorn[standard]

In summer, httptools upgraded to version `0.3.*` as a maintainace release (no functional changes), just changing the packaging and wheel building to support Python 3.10, dropping Python 3.5 support, see here: https://github.com/MagicStack/httptools/releases/tag/v0.3.0



Currently, the `uvicorn[standard]` dependencies pin `httptools` at version `0.2.*`:

https://github.com/encode/uvicorn/blob/a7d26bb76c29a9bd12f53b3a73ae5614592cdfb4/setup.py#L59



This prevents `uvicorn[standard]` to use wheels for `httptools `on Python 3.10 (cp310), **often preventing the simple usage of uvicorn[standard] with Python 3.10**, as they are only available on `httptools>=0.3.0`.



**Proposed fix:**

Either use `httptools=0.3.x` or drop the pin (initial reason for pinning?)



**Explanation** 

As the `httptools` upgrade is just a packaging release and Python 3.5 is not supported by `uvicorn `anyways, upgrading the pinning to 0.3.x (or dropping it entirely, after careful consideration) should not introduce any side-effects or breaking changes for the supported user base, but instead enable the simple usage of Python 3.10 without the need to manually build wheels for `httptools` on Python 3.10 during `pip install`



I did not open a pull request yet as I don't know if the pin is intentional and wanted to discuss this first.

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