fix(tox.ini): set build dir to match where CI jobs expect it to be

Although we write code coverage information to build/coverage.xml, our CI jobs expect to find it at build/py3/coverage.xml. Update tox.ini to match.

Aside: I chose to fix the tox.ini to match the CI jobs rather than the other way around because we definitely want different tox envs to write artefacts to different directories rather than colliding within the same build directory.

Looking at the test log for this MR confirms that junit and coverage artifacts have been uploaded:

Uploading artifacts...
/builds/uis/devops/iam/activate-account/api/tox-artifacts/: found 38 matching artifact files and directories 
Uploading artifacts as "archive" to coordinator... 201 Created  id=1802457 responseStatus=201 Created token=glcbt-64
Uploading artifacts...
/builds/uis/devops/iam/activate-account/api/tox-artifacts/py3/junit.xml: found 1 matching artifact files and directories 
Uploading artifacts as "junit" to coordinator... 201 Created  id=1802457 responseStatus=201 Created token=glcbt-64
Uploading artifacts...
/builds/uis/devops/iam/activate-account/api/tox-artifacts/py3/coverage.xml: found 1 matching artifact files and directories 
Uploading artifacts as "cobertura" to coordinator... 201 Created  id=1802457 responseStatus=201 Created token=glcbt-64

Closes #9 (closed)