Add support for LTR390

[sjtrny]

Description:

Adds support for the LTR390 sensor, which is a UV and ambient light sensor.

Pull request in esphome-docs with documentation (if applicable): esphome/esphome-docs#979

Checklist:

• The code change is tested and works locally.
• Tests have been added to verify that the new code works (under tests/ folder).

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in esphome-docs.

[probot-esphome]

Hey there @OttoWinter, mind taking a look at this pull request as its been labeled with an integration (api) you are listed as a codeowner for? Thanks!
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[OttoWinter]

Thanks for this contribution! Sorry for the long review delay.

I just took a look at this PR, and found some minor things to address - I figured it's probably easier if I just push the changes to make it quicker.

Can you check if the component works with those changes? Thanks!

[probot-esphome]

Hey there @esphome/core, mind taking a look at this pull request as it has been labeled with an integration (i2c) you are listed as a code owner for? Thanks!
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[sjtrny]

@OttoWinter I've tested changes locally and everything seems ok!

[oxan]

@OttoWinter I've tested changes locally and everything seems ok!

Okay, it seems we can merge this then :) I've removed one change to the I2C component that, while it seems correct, probably ended up in this branch by accident.

[jesserockz]

Okay, it seems we can merge this then :) I've removed one change to the I2C component that, while it seems correct, probably ended up in this branch by accident.

Otto pushed that change into this PR (a8a14c7)

[probot-esphome]

Hey there @esphome/core, mind taking a look at this pull request as it has been labeled with an integration (i2c) you are listed as a code owner for? Thanks!
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[oxan]

Otto pushed that change into this PR (a8a14c7)

Ah, apparently I read over the part where he said he'd change the constructor visibility. Added it back in (though I do think changes like that are better done in separate PR's).