Badkeys

* Supports scanning DKIM keys both in files (e.g. zone files,
output of tools like dig/host) or directly from DNS
(requires dnspython).

* Silence deprecation warnings for SSH DSA keys, we may need
to implement our own parser in the future.

* Add __version__ module variable and --version command line.

Major speedup release:
* The rsainvalid module contained a check whether the N in an RSA key was prime, which would indicate a defect key. Via profiling with cProfile/snakeviz, I discovered that this check is very computationally expensive. Removing this feature, which is not very valuable, makes badkeys 10x faster.

Minor changes:
* Continue TLS scans when remote server drops packages.
* Produce proper error message in SSH scan mode when paramiko is not available.
* Make blocklist tests optional (for distro build systems).

* This version introduces a new module to detect keys used in the xz backdoor. The backdoor expects a certain type of RSA public key, badkeys can detect these now. This detection can theoretically lead to false positives, however, the false positive rate is low enough (~1:2^62) that it will likely never show up in practice.
* Move from setup.py/setup.cfg to pyproject.toml.
* Avoid installing the "tests" dir in the wheel.
* Avoid open_binary() deprecation warning and use new files() API. This raises the required Python version to 3.9.