improve portability of reproducible tarballs by replacing external tar command with tarfile module

[lexming]

fixes #4657

• ~~use more portable --date argument for touch~~
• ~~catch failed commands inside the pipeline~~
• ~~move generation of command to make reproducible archives intro its own method~~
• ~~replace harcoded pattern in tests of reproducible archives command for call to filetools.reproducible_archive_cmd~~
• make new implementation of reproducible_archive_cmd using the tarfile module
• added new filetools.make_archive() method and related unit test
• change handling of filename argument in filetools.get_source_tarball_from_git() to expect filenames without extension
  • archive extension is appended as needed by make_archive()
  • known archive extensions are automatically removed from filename if it contains one
• add required argument to filetools.find_extensions()