setup.py: set version to 1.20.0rc3.

Also updated minimum python version from 3.6 to 3.7 to match the wheels we
build.

Author: julian-smith-artifex-com

setup.py

@@ -673,7 +673,7 @@ def get_mupdf():
 
 setup(
     name="PyMuPDF",
-    version="1.20.0rc2",
+    version="1.20.0rc3",
     description="Python bindings for the PDF toolkit and renderer MuPDF",
     long_description=readme,
     long_description_content_type="text/markdown",
@@ -683,7 +683,7 @@ def get_mupdf():
     author_email="[email protected]",
     cmdclass={"build_py": build_ext_first},
     ext_modules=[module],
-    python_requires=">=3.6",
+    python_requires=">=3.7",
     py_modules=["fitz.fitz", "fitz.utils", "fitz.__main__"],
     license="GNU AFFERO GPL 3.0",
     project_urls={