Prepare release 1.0.1 (Android: cache Python dist tarballs)

[FeodorFitsner]

Summary

Fixes flet-dev/flet#6555 — flet debug android was re-downloading the three python-android-dart-3.12-<abi>.tar.gz tarballs (~30–100 MB each) on every build, often the dominant cost of a debug cycle.

• The downloadDistArchive_* Gradle tasks in serious_python_android now write to a persistent cache dir — $FLET_CACHE_DIR/python-build/v<python_version>/ if set, otherwise ~/.flet/cache/python-build/v<python_version>/ — and run with onlyIfModified true + useETag "all" + tempAndMove true. Subsequent builds issue a conditional GET (If-None-Match / If-Modified-Since) against objects.githubusercontent.com instead of re-downloading the full tarball, so when the upstream release re-publishes a tarball at the same versioned URL (e.g. a Python patch update under the existing v3.12 release) the validators flip and the cache refreshes automatically; otherwise the build skips the download entirely. tempAndMove guards against partial downloads being kept in the cache.
• All packages bumped to 1.0.1 (serious_python, serious_python_android, serious_python_darwin, serious_python_linux, serious_python_windows, serious_python_platform_interface) with identical changelog entries — matches the monorepo-style coordinated-release pattern used for 1.0.0.
• The 1.0.1 changelog also rolls in the PIP_REQUIRE_VIRTUALENV=false fix from Define PIP_REQUIRE_VIRTUALENV=false to make package command work in some environments #202 / Set PIP_REQUIRE_VIRTUALENV=false to make package command work in restricted environments #204 (merged on main after 1.0.0 but never written into a release note).
• Adds **/.cxx/ to the three example android/.gitignore files so Gradle CMake build output stops showing up as untracked noise during local testing.

The matching flet CLI side (build-template caching + FLET_CACHE_DIR env propagation + serious_python pin bump to 1.0.1) will be opened as a separate PR against flet-dev/flet.

Test plan

• Cold run: delete ~/.flet/cache/ and run flet debug android -vv against a sample app — confirm the three tarballs land under ~/.flet/cache/python-build/v3.12/ and the build succeeds.
• Warm run: run the same command again — Gradle logs should show downloadDistArchive_* tasks completing in ~1 round-trip per ABI (a 304) instead of full downloads; end-to-end wall time drops by minutes.
• Re-publish invalidation: hand-edit the gradle-download-task .etag / .lastModified sidecars next to a cached tarball to a stale value, re-run, and confirm the conditional GET issues a re-download.
• FLET_CACHE_DIR override: FLET_CACHE_DIR=/tmp/flet-cache flet debug android — confirm tarballs land under /tmp/flet-cache/python-build/v3.12/.
• Offline with warm cache: kill network, run again with a populated cache — build should still succeed.

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