MINOR: [CI] Set max-parallel=20

[rok]

Rationale for this change

We want to comply with ASF policy

What changes are included in this PR?

Sets max-parallel=20 in .github/workflows/python.yml

Are these changes tested?

No.

Are there any user-facing changes?

Only to CI users.

[raulcd]

LGTM but I am wondering why this has been flagged here as a problem but not on other workfows like the CPP one:

arrow/.github/workflows/cpp.yml

Lines 79 to 104 in e11aeee

	strategy:
	fail-fast: false
	matrix:
	include:
	- arch: amd64
	clang-tools: 18
	image: conda-cpp
	llvm: 18
	runs-on: ubuntu-latest
	simd-level: AVX2
	title: AMD64 Conda C++ AVX2
	ubuntu: 22.04
	- arch: amd64
	clang-tools: 18
	image: ubuntu-cpp-sanitizer
	llvm: 18
	runs-on: ubuntu-latest
	title: AMD64 Ubuntu 24.04 C++ ASAN UBSAN
	ubuntu: 24.04
	- arch: arm64v8
	clang-tools: 18
	image: ubuntu-cpp
	llvm: 18
	runs-on: ubuntu-24.04-arm
	title: ARM64 Ubuntu 22.04 C++
	ubuntu: 22.04

Seems slightly arbitrary. @assignUser any idea?

[assignUser]

@raulcd I assume it's the way the check script calculates the parallelism not properly handeling include: but I don't know

[kou]

+1