Prepare 0.18.0 release

[lukpueh]

Update CHANGELOG and setup.py for an imminent 0.18.0 release blocks on #300

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securesystemslib v0.18.0

Added

• interface.generate_and_write_unencrypted_{rsa,ed25519,ecdsa}_keypair (Consolidate key interface password handling and general overhaul #288)
• interface.generate_and_write_{rsa,ed25519,ecdsa}_keypair_with_prompt (Consolidate key interface password handling and general overhaul #288)
• interface.import_privatekey_from_file(Consolidate key interface password handling and general overhaul #288)
• GitHub Action to auto-check upstream changes for vendored ed25519 (github: Add workflow to check ed25519 upstream #294)

Changed

• interface.generate_and_write_{rsa,ed25519,ecdsa}_keypair require a password
  as first positional argument (Consolidate key interface password handling and general overhaul #288)
• interface.import_{rsa,ed25519,ecdsa}_privatekey_from_file do not error on
  empty password, but pass it on to lower level decryption routines (Consolidate key interface password handling and general overhaul #288)
• interface.import_ecdsa_privatekey_from_file supports loading unencrypted
  private keys (Consolidate key interface password handling and general overhaul #288)
• Revise interface and gpg.functions docstrings and example snippets and
  use Sphinx compatible Google Style docstring format (Consolidate key interface password handling and general overhaul #288, Adopt interface changes (#288) in README snippets #300)
• Linter-flagged cosmetic changes (Minor changes #292, More cosmetic changes – remove unused variables, unused imports and change comparisons to None #295, Remove invalid escape sequence from docstring #296)
• Bump dependencies: cryptography (Bump cryptography from 3.1.1 to 3.2 #291, build(deps): bump cryptography from 3.2 to 3.2.1 #293)
• Bump vendor copy of ed25519 (ed25519: Update to upstream master HEAD #299)

[coveralls]

Coverage increased (+0.0005%) to 98.785% when pulling c80a1c9 on lukpueh:bump_version_0.18.0 into 92da669 on secure-systems-lab:master.

[coveralls]

Coverage remained the same at 98.785% when pulling b37d453 on lukpueh:bump_version_0.18.0 into deb50ad on secure-systems-lab:master.

[joshuagl]

Other than a minor suggestion to use some extra punctuation on one of the changelog entries, LGTM.

[lukpueh]

Thanks, @joshuagl! I squashed your suggestion in and rebased on top of the updated master that now includes #300. I think this is good to go. Although I will wait with the merge until tomorrow when I finalize the release.