build: disable building of epub on readthedocs.org

The epub build fails because we have `sphinx.fail_on_warning` set and
the epub builder does not know how to handle `html_favicon = "_static/RDFlib.ico"`
from `docs/conf.py`, which causes this warning:

```
WARNING: unknown mimetype for _static/RDFlib.ico, ignoring
```

I created an upstream issue for this: sphinx-doc/sphinx#10350

Also:
- build: add `.pytest_cache` to `.dockerignore` - mainly because it is
  quite big and it is not needed when building the test Dockerfile.

.dockerignore

@@ -1,4 +1,5 @@
 .tox
 .venv
 .mypy_cache
+.pytest_cache
 .git

.readthedocs.yaml

@@ -1,7 +1,13 @@
 ---
 # https://docs.readthedocs.io/en/stable/config-file/v2.html
 version: 2
-formats: all
+# NOTE: not builing epub because epub does not know how to handle .ico files
+# which results in a warning which causes the build to fail due to
+# `sphinx.fail_on_warning`
+# https://github.com/sphinx-doc/sphinx/issues/10350
+formats:
+  - htmlzip
+  - pdf
 
 build:
   os: ubuntu-20.04