Run non-crypto CLI commands with strict=False

Description of the changes

Turn off strict mode in non-cryptographic CLI commands. Being very particular about semantics isn't as big of a deal when there's no cryptographic commitment involved.

Fixes #262.

Checklist

Please go over this checklist to increase the chances of your PR being worked on in a timely manner. Deviations are allowed with proper justification (see previous section).

[x] I have read the project's CoC and contribution guidelines.
[x] I understand and agree to the terms in the Developer Certificate of Origin as applied to this contribution.
[x] All new code in this PR has full test coverage.

For bug fixes (delete if not applicable)

[x] My changes do not affect any public API or CLI semantics.
[x] My PR adds regression tests (i.e. tests that fail if the bug fix is not applied).
[x] All new code in this PR has full test coverage.

Additional comments

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Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (79b64d7) 98.13% compared to head (e22f81a) 98.13%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #264 +/- ## ======================================= Coverage 98.13% 98.13% ======================================= Files 104 104 Lines 14535 14535 ======================================= Hits 14264 14264 Misses 271 271 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `98.13% <100.00%> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matthias+Valvekens#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://app.codecov.io/gh/MatthiasValvekens/pyHanko/pull/264?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matthias+Valvekens) | Coverage Δ | | |---|---|---| | [pyhanko/cli/commands/fields.py](https://app.codecov.io/gh/MatthiasValvekens/pyHanko/pull/264?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matthias+Valvekens#diff-cHloYW5rby9jbGkvY29tbWFuZHMvZmllbGRzLnB5) | `100.00% <100.00%> (ø)` | | | [pyhanko/stamp.py](https://app.codecov.io/gh/MatthiasValvekens/pyHanko/pull/264?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matthias+Valvekens#diff-cHloYW5rby9zdGFtcC5weQ==) | `98.70% <100.00%> (ø)` | |

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MatthiasValvekens / pyHanko