Update README.md to list Python as required (#753)

### Description
Python is required to compile Quokka, since we need to run a Python
script in Microphysics to generate some header files.

### Related issues
N/A

### Checklist
_Before this pull request can be reviewed, all of these tasks should be
completed. Denote completed tasks with an `x` inside the square brackets
`[ ]` in the Markdown source below:_
- [x] I have added a description (see above).
- [ ] I have added a link to any related issues see (see above).
- [x] I have read the [Contributing
Guide](https://github.com/quokka-astro/quokka/blob/development/CONTRIBUTING.md).
- [ ] I have added tests for any new physics that this PR adds to the
code.
- [ ] I have tested this PR on my local computer and all tests pass.
- [ ] I have manually triggered the GPU tests with the magic comment
`/azp run`.
- [x] I have requested a reviewer for this PR.

README.md

@@ -28,12 +28,11 @@ Quokka also features advanced Adaptive Quokka Refinement:tm: technology:
 ## Dependencies
 * C++ compiler (with C++17 support)
 * CMake 3.16+
+* Python 3.8+
 * MPI library with GPU-aware support (OpenMPI, MPICH, or Cray MPI)
 * HDF5 1.10+ (serial version)
 * CUDA 11.7+ (optional, for NVIDIA GPUs)
 * ROCm 5.2.0+ (optional, for AMD GPUs)
-* Ninja (optional, for faster builds)
-* Python 3.7+ (optional)
 * ADIOS2 2.9+ with GPU-aware support (optional, for writing terabyte-sized or larger outputs)
 
 ## Problems?