Fix CI

[bachya]

Describe what the PR does:

For some reason, the latest version of pytest won't work unless we specify the tests/ directory; without it, CI breaks. This PR fixes it.

Does this fix a specific issue?

Fixes https://github.com/bachya/aioambient/issues/

Checklist:

• Confirm that one or more new tests are written for the new functionality.
• Update README.md with any new documentation.
• Add yourself to AUTHORS.md.

[codeclimate]

Code Climate has analyzed commit 548dd37 and detected 0 issues on this pull request.

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[codecov]

Codecov Report

Merging #16 into dev will not change coverage.
The diff coverage is n/a.

@@         Coverage Diff         @@
##            dev    #16   +/-   ##
===================================
  Coverage   100%   100%           
===================================
  Files         8      8           
  Lines       142    142           
===================================
  Hits        142    142

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