support SOCKS proxies in sentry_sdk

[Roguelazer]

Fixes #1049

Not sure that this is the cleanest solution, but it sure seems to work.

[kenoss]

This PR is very useful. I'm not a maintainer of this repo but I'll note some points I found.

1. About PySocks dependency.

urllib3 does not install PySocks by default.
In other words, after this PR, the users need to write dependencies both sentry_sdk and PySocks explicitly and separately, and urllib3 can't control what version of PySocks is installed.
setry_sdk uses extra_require already, and (AFAIK) it is better to do it.
(But, a little tricky. sentry_sdk[socks] should trigger urllib3[socks] rather than PySocks.)
Users will be able to install by pip install "sentry_sdk[socks]".

2. About test.

This PR would be more great if it has tests, like https://github.com/getsentry/sentry-python/pull/838/files I guess.

[github-actions]

This pull request has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you label it Status: Backlog or Status: In Progress, I will leave it alone ... forever!

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"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀

[Roguelazer]

reset the clock