Save the setup key and config files with owner-only permissions

[aharoitx]

Summary

bitpay_setup.py saved two files with the default mode (usually 0644), so any local user could read them:

• private_key.pem: the merchant's private key.
• bitpay.config.json: the API tokens, and the private key too when the "plain text" option is used.

Both files are now saved with mode 0600 (only the owner can read and write).

Changes

• New helper write_secret_file in src/bitpay/utils/secret_file.py. It creates the file with 0600 and runs chmod before and after writing. The mode given to os.open only applies to new files, so chmod also fixes files left by an older setup run.
• bitpay_setup.py uses the helper for both files.
• New unit tests in tests/unit/utils/test_secret_file.py.

On Windows, chmod only controls the read-only flag, so this change does nothing there. The tests skip on Windows.

Testing

• black --check src/, mypy src/, mypy -p src: OK
• pytest -m unit: 96 passed (93 existing + 3 new)
• Manual: ran the setup with umask 022. Before: both files 0644. After: both 0600, also when the files already existed with 0644.
• The client still loads the generated config and key (Client.create_client_by_config_file_path).

Jira: IM-191