Since the default arguments have no action, an optional args doesn't
make sense.

Fallback to long id GPG key lookup when fingerprint is not available in signature. Enable gpgv1 signatures to work. (Closes: 120)

GnuPG(): re-order verify_sig statements, fix typo in comment and remove default assignment =[] from get_keys_data signature

Rename GnuPG.add_signature() to GnuPG.import_key(). Remove the capability to import key from file (not used in debexpo)

- Add two new classes:
  - VirtualKeyring, that builds a temporary keyring from a user or a
    standalone key
  - GPGSignedFile, that handle gpg signed files
- Add more exceptions to be precise in the error
- Use --status-fd to get machine-readable data from gpg
- Use GNUPGHOME to create temporary secure keyrings
- Remove some unused methods and classes (GPGAlgo is now in DB)

Currently, gpg keys with multiple uid won't work for the following use
case:

- Package import will fail as the regexp matching gpg uids only match the
primary uid.

- Only the primary uid will be matched using uid fingerprint on user
profile update:

gpg --with-colons < key.gpg fails to print uid fingerprint
gpg --with-colons --import --import-options show-only < key.gpg succeed

https://unix.stackexchange.com/questions/335669/gnupg-2-1-16-with-fingerprint-no-longer-works-to-show-fingerprints

KeyData() and Uid() class imported from nm.debian.org code.
Adding class GPGAlgo() to output old id format present in gnupgv1.

(gpg_path's validity check, 'is_valid' default value)

 - use --status-fd for easier parsing of gpg commands' result
 - add a default gpg path ('/usr/bin/gpg') and keyring ('~/.gnupg/keyring.gpg')
 - verify_file can take data as a string too

(+ add generic exception for missing data in a gpg command)

(Was working only with one-uid keys.)