Update boto3 requirement from <1.15.0,>=1.9.0 to >=1.9.0,<1.16.0

Updates the requirements on [boto3](https://github.com/boto/boto3) to permit the latest version.
- [Release notes](https://github.com/boto/boto3/releases)
- [Changelog](https://github.com/boto/boto3/blob/develop/CHANGELOG.rst)
- [Commits](boto/boto3@1.9.0...1.15.0)

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

requirements-optional.txt

@@ -4,7 +4,7 @@ aeidon>=1.6.0,<1.8.0
 # Antispam
 akismet>=1.0.1,<1.2
 # Amazon
-boto3>=1.9.0,<1.15.0
+boto3>=1.9.0,<1.16.0
 # Unicode
 chardet==3.0.4
 # LDAP