add file marker for type checking (#1598)

I think this is the correct way to do this based on my understanding of
the guide to making a [pep561 compliant
package](https://mypy.readthedocs.io/en/stable/installed_packages.html#creating-pep-561-compatible-packages)
and this example in
[black](https://github.com/psf/black/pull/1395/files). Given that I get
all the type-checking info from a local installation, I don't know if
this is testable until it's published on Pypi.

resolves #1210

photon-lib/py/photonlibpy/py.typed

@@ -0,0 +1 @@
+