build: Teach twine that our README.md is in Markdown

Right now, `twine` complains that our README’s Markdown is improperly formatted
as ReStructuredText.  This patch tells it explicitly that the README is
formatted in GitHub Flavored Markdown, by reading the `README.md` file into the
`long_description` field explicitly and then explicitly selecting the correct
content type.

Signed-off-by: Patrick M. Niedzielski <[email protected]>

setup.py

@@ -17,6 +17,7 @@
 import platform
 import sys
 
+from pathlib import Path
 import pkgconfig
 from setuptools import Extension
 from setuptools import setup
@@ -181,6 +182,10 @@ def create_extension(name, libraries, **kwargs):
 with open("src/blazingmq/_about.py") as fp:
     exec(fp.read(), about)
 
+# Read the contents of the README file.
+this_directory = Path(__file__).parent
+long_description = (this_directory / "README.md").read_text()
+
 setup(
     name="blazingmq",
     description="Python BlazingMQ API",
@@ -204,6 +209,8 @@ def create_extension(name, libraries, **kwargs):
     package_data={"blazingmq": ["py.typed", "_ext.pyi"]},
     package_dir={"": "src"},
     packages=["blazingmq"],
+    long_description=long_description,
+    long_description_content_type="text/markdown",
     ext_modules=cythonize(
         EXTENSIONS,
         include_path=["src/declarations"],