Add *SPDX* blocks in all existing source files

[Mischback]

• As of now, only the test suite and the related validator is implemented
  • Add the following block to all (Python) source files

    # SPDX-FileCopyrightText: 2022 Mischback
    # SPDX-License-Identifier: MIT
    # SPDX-FileType: SOURCE

• SPDX-FileType may be one of
  • https://spdx.github.io/spdx-spec/file-information/#8.3
  • SOURCE
  • DOCUMENTATION
  • OTHER