Replace filter/lambda with for loop in gpg.common
Replace filter/lambda with for loop in gpg.common

Pylint complains about lambda functions, and suggests a list
comprehension instead.
```
************* Module in_toto.gpg.common
W:184,10: map/filter on lambda could be replaced by comprehension (deprecated-lambda)
```

Our code style guideline prefers simple for loops over both:
https://github.com/secure-systems-lab/code-style-guidelines#no-lambda-functions-or-lisp-esque-code

Performance should not be an issue in this case. So let's use a
for loop.

secure-systems-lab/securesystemslib,secure-systems-lab/securesystemslib