Cleaner no proxy support (#1103)

Cleaner no proxy support (#1103)

* Add internal URLMatcher class

* Use URLMatcher for proxy lookups in transport_for_url

* Docstring

* Pin pytest

* Add support for no-proxies configurations

* Don't call should_not_proxy on each request

* Drop print statements

* Tweak comment

* Tweak comment on domain wildcards

* Update httpx/_utils.py

Co-authored-by: Florimond Manca <florimond.manca@gmail.com>
* Pull test_should_not_be_proxied cases into test_proxies_environ

Co-authored-by: Florimond Manca <florimond.manca@gmail.com>