Add py.typed to enable downstream type usage

[gadomski]

Related Issue(s):

• Closes Provide stub file for mypy integration #143

Description:

Based on the instructions at https://mypy.readthedocs.io/en/stable/installed_packages.html#making-pep-561-compatible-packages. Manual checkout of downstream usage:

$ mkvirtualenv test-project >/dev/null 2>&1
$ pip install -U pip pystac-client mypy >/dev/null 2>&1
$ cat foo.py
from pystac_client.client import Client

id = 2
client = Client(id)
$ mypy foo.py
foo.py:1: error: Skipping analyzing "pystac_client.client": module is installed, but missing library stubs or py.typed marker
foo.py:1: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
Found 1 error in 1 file (checked 1 source file)
$ pip uninstall -y pystac-client >/dev/null 2>&1
$ pip install 'git+https://github.com/stac-utils/pystac-client@issues/143-mypy-stubs' >/dev/null 2>&1
$ mypy foo.py
foo.py:4: error: Missing positional argument "description" in call to "Client"
foo.py:4: error: Argument 1 to "Client" has incompatible type "int"; expected "str"
Found 2 errors in 1 file (checked 1 source file)

PR Checklist:

• Code is formatted
• Tests pass
• Changes are added to the CHANGELOG