Fix not implemented error

[Inveracity]

Reason for the change
closes #201

Description
Dynamically importing using the imp library causes setuptools and/or pyinstaller to not find the async libraries.

I also could not get the pyinstaller --hidden-import "rethinkdb.net_asyncio" to work, probably for the same reason.

With these changes the behavior is exactly the same, except now the async libraries are recognised as packages and pyinstaller can create a binary that works.

I appreciate that this change makes the code less dynamic whenever a new async library gets implemented. Maybe there's a better fix? Or maybe pyinstaller will eventually support your way of structuring the rethinkdb package, but this definitely made it work for me.

Tested with

• OS: Windows 10
• Python version: 3.8.2

Checklist

• I have read and agreed to the RethinkDB Contributor License Agreement

[gabor-boros]

For first review this seems good. Let me check it tomorrow ☺️😇

[gabor-boros]

I like that this is a bit more explicit. Also it is easier to see what’s happening. @lsabi would you like to have a second look on this PR? If not I’d merge this and cherry-pick to the python 2-3 branch with the other merged PR

[lsabi]

I would capture the possibility that a library name passed as parameter is not valid.

Basically, I would move the first if statement down and add a second check.
It would become something like

`if library is None or self.connection_type is None:

self.connection_type = self.net.DefaultConnection`

Better safe than sorry

[gabor-boros]

Yep, that’s true tho

[gabor-boros]

@Inveracity Could you please adjust the code accordingly? 😇🙏

[gabor-boros]

Here is an overview of what got changed by this pull request:

Issues
======
+ Solved 1
           

Complexity increasing per file
==============================
- rethinkdb/__init__.py  5
         

See the complete overview on Codacy

[Inveracity]

Thanks for the review!

I hope I made the correct change requested by lsabi 😄

[lsabi]

Looks good to me

[gabor-boros]

LGTM too. I'll check the changes with some more tests and merge this PR 😊
In case the tests are passing I'll make sure this change got released.

[Inveracity]

Thank you so much!

[McSinyx]

@gabor-boros, could you please cut a release with this included? import pkg_resources in 2.4.7 is breaking isolated build using flit.

[gabor-boros]

@McSinyx Sure, I'll cut a new release today.

[McSinyx]

Thanks!