Expose defaults from _config to users

[Bluenix2]

I am using HTTPX inside of a wrapper class and I want to expose some of the configuration options HTTPX has (timeouts). The issue is that I would like to retain the defaults that HTTPX uses.

Python does not make it easy to use a functions defaults when calling it unless you know at compile-time which parameters will be passes. The only option is dynamically building a dictionary and unpacking it - not a nice solution.

Therefore I think that HTTPX should expose the following constants used as defaults:

httpx/httpx/_config.py

Lines 361 to 363 in 6f31bc4

	DEFAULT_TIMEOUT_CONFIG = Timeout(timeout=5.0)
	DEFAULT_LIMITS = Limits(max_connections=100, max_keepalive_connections=20)
	DEFAULT_MAX_REDIRECTS = 20

I have two options currently: either copy in the code above, or import directly from the file. Neither of which is perfectly satisfying.

[acostapazo]

It will be fantastic to have something to configure global defaults.

This is my proposal using envars.

import os

DEFAULT_TIMEOUT_CONFIG = Timeout(timeout=float(os.getenv("HTTPX_DEFAULT_TIMEOUT", 5.0)))
DEFAULT_LIMITS = Limits(
    max_connections= int(os.getenv("HTTPX_DEFAULT_LIMITS_MAX_CONNECTIONS", 100)),
    max_keepalive_connections=int(os.getenv("HTTPX_DEFAULT_LIMITS_MAX_KEEPALIVE_CONNECTIONS", 20))
) 
DEFAULT_MAX_REDIRECTS = int(os.getenv("HTTPX_DEFAULT_MAX_REDIRECTS", 20))