docs: remove sponsor, correct headline

Author: dbanty

.github/sponsors/fern.png

@@ -7,7 +7,7 @@
 
 # openapi-python-client
 
-Generate modern Python clients from OpenAPI 3.x documents.
+Generate modern Python clients from OpenAPI 3.0 documents.
 
 _This generator does not support OpenAPI 2.x FKA Swagger. If you need to use an older document, try upgrading it to
 version 3 first with one of many available converters._
@@ -22,10 +22,6 @@ This tool focuses on creating the best developer experience for Python developer
 2. Having documentation and usage instructions specific to this one generator.
 3. Being written in Python with Jinja2 templates, making it easier to improve and extend for Python developers. It's also much easier to install and use if you already have Python.
 
-## Sponsors
-
-<a href="https://www.devmark.ai/fern/?utm_source=openapi-python-client&utm_loc=readme&utm_type=logo" target="_blank" title="Fern | SDKs and API docs"><img src="https://raw.githubusercontent.com/openapi-generators/openapi-python-client/main/.github/sponsors/fern.png"></a>
-
 ## Installation
 
 I recommend you install with [pipx](https://pipxproject.github.io/pipx/) so you don't conflict with any other packages you might have: `pipx install openapi-python-client --include-deps`.