Bump boto3 from 1.15.1 to 1.15.6 (#1831)

Bumps [boto3](https://github.com/boto/boto3) from 1.15.1 to 1.15.6.
- [Release notes](https://github.com/boto/boto3/releases)
- [Changelog](https://github.com/boto/boto3/blob/develop/CHANGELOG.rst)
- [Commits](https://github.com/boto/boto3/compare/1.15.1...1.15.6)

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements.txt

@@ -1,5 +1,5 @@
 adr==0.19.0
-boto3==1.15.1
+boto3==1.15.6
 imbalanced-learn==0.7.0
 joblib==0.16.0
 libmozdata==0.1.68