Fixes for capnp 1.0 (#1)

* add capnp_api.h to gitignore

* Change type of read_min_bytes from size to int

Not sure why this was not causing issues before or if that
is the right fix ... but it seems to be fine :)

* Adapt python_requires to >=3.8

This was overlooked when 3.7 was deprecated. The ci no longer
works with python 3.7 and cibuildwheel uses python_requires ...

* Replace deprecated find_module with find_spec (importlib)

find_module was deprecated with python 3.4 and python 3.12
removed it (https://docs.python.org/3.12/whatsnew/3.12.html#importlib).

The new command is find_spec and only required a few adaptions

CHANGELOG.md

@@ -1,6 +1,6 @@
 ## v2.0.0b1 (2023-10-03)
 - Update to bundled capnproto-1.0.1
-- Remove explicit support for Python 3.7 (though wheels are still built for now)
+- Remove support for Python 3.7
 - Use custom build backend to support build args (#328)
 - Update Cython version and Python to 3.12 (#320)
 - Wrap all capnp code in a context-manager to avoid segfaults (#317)