🔥 Remove Z3 from build dependencies (#199)

## Description

This PR gets rid of the Z3 build dependency. Due to build isolation, the
virtual environment containing Z3 (that is being linked to the C++
binding library) does no longer exist after the build. Hence, the
package install is essentially broken if the Z3 Python package is picked
up during the build.

This was a workaround for getting the RtD build to work. I managed to
work around the underlying issue by figuring out a way to specify the
`Z3_ROOT` environment variable for the package installation step.

## Checklist:

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your pull request will be merged swiftly.
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- [x] The pull request only contains commits that are related to it.
- [x] I have added appropriate tests and documentation.
- [x] I have made sure that all CI jobs on GitHub pass.
- [x] The pull request introduces no new warnings and follows the
project's style guidelines.

Signed-off-by: Lukas Burgholzer <lukas.burgholzer@jku.at>

.readthedocs.yaml

@@ -7,16 +7,11 @@ submodules:
 build:
   os: ubuntu-22.04
   tools:
-    python: "3.9"
-  apt_packages:
-    - cmake
+    python: "3.10"
+  jobs:
+    post_create_environment:
+      - pip install "z3-solver>=4.8.15"
+      - Z3_ROOT=$(python -c "import z3 as _; print(_.__path__[0])"); export Z3_ROOT; pip install -e .[docs] -v
 
 sphinx:
   configuration: docs/source/conf.py
-
-python:
-  install:
-    - method: pip
-      path: .
-      extra_requirements:
-        - docs

pyproject.toml

@@ -4,7 +4,6 @@ requires = [
     "setuptools_scm[toml]>=7",
     "ninja>=1.10; sys_platform != 'win32'",
     "cmake>=3.19",
-    "z3-solver>=4.8.15,<4.12.0",
 ]
 build-backend = "setuptools.build_meta"