Add PEP-561 type-support marker

[nemacysts]

This will tell mypy and other tooling that this package is typed and that the types in this library (when present) should be used.

Once we start using a version with this in paasta, we'll unearth two failing type-checks:

paasta_tools/tron_tools.py:354: error: Argument "extra_volumes" to "get_spark_conf" of "SparkConfBuilder" has incompatible type "List[DockerVolume]"; expected "Optional[List[Mapping[str, str]]]"
paasta_tools/cli/cmds/spark_run.py:1236: error: Argument "extra_volumes" to "get_spark_conf" of "SparkConfBuilder" has incompatible type "List[DockerVolume]"; expected "Optional[List[Mapping[str, str]]]"

but:
a) these are easy to fix and,
b) it's a good thing that we're now using the right types :p