Add pack_command to support writing via hiredis-py

[prokazov]

As of now, hiredis-py only exposes Reader functionality, that helps to speed up the RESP deserialization.
pack_command serializes redis-py command (tuple of str, bytes, int or float) to a bytes object with the RESP representation of the command.

[chayim]

Big fan, very small changes. Any chance you've finished up on the redis-py PR we were discussing that shows covered usage?

[prokazov]

Any chance you've finished up on the redis-py PR ...
I'll publish a draft PR soon but as of now:
redis/redis-py@master...prokazov:redis-py:master
It's kind of non-trivial to properly test redis-py against unpublished hiredis-py.

[prokazov]

I mean I tested the redis-py with the new pack_command locally, but its CI/CD actions target the stock hiredis-py.
The changes above only cover the regular ("sync") API. The async one would be basically a "copy-paste" of the sync API (with a review feedback). For some reason the async redis-py follows that powerful design pattern.

[chayim]

@prokazov Want to tag me and @dvora-h there... we can continue there. This looks good. Once CI reports in, etc, I'll prep a release - for today or Sunday.

[chayim]

Changes, as per Bsymbolic build fails

[zalmane]

@chayim I pushed a commit with conditional flags. Can you re-run the CI? Are there any wheels built not covered by the CI? I excluded win and darwin platforms which don't support the flag. The CI failure of the ubuntu on 3.7 seemed unrelated.

[chayim]

Looks great. Given a past bug, I just added #148. I'll merge that into your branch, and see it through. If we're good there, we're ready to merge!