Mar

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* Dropped python3.6 support
* Added python3.9 and python3.10 support
* Fixed file descriptor leaks

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* Added new autograph stage public key
* Dropped python3.5 support

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* Support extracting mar hash for external signing, and injecting signatures
into unsigned files.
* File modes are set on extraction
* `mar -v` can now verify that unsigned mar files are well formed
* Added helper productinfo property to MarReader to allow convenient access to
the product information information
* Internal signing API changed:
* Got rid of Verifier/Signer classes
* Internal API for the mar format changed:
* offets added for the beginning and end of signature and additional blocks
* correctly represent additional section padding
* Support MAR files without additional sections

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* Remove deprecated usage of signature generation and verification in
cryptography library. This changes the type of object returned by the
make_signer and make_verifier functions.

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* Update dependencies; dependencies are now also pinned in setup.py
* Correct the key size in exception messages
* Fix for construct 2.8.22 and higher
* Fix for hypothesis 3.44.16

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* Fix bug when writing MAR files: the index size was incorrect
* Support writing interable streams
* Add new Mozilla SHA384 public keys