Fix slicing of energy/wavenumber signal with isig

[jlaehne]

Description of the change

Fixes #155.

.isig[] was failing on signals converted using to_eV() or to_invcm().

For tests to run, deprecated --use-feature=2020-resolver had to be removed from PIP_ARGS in workflows/tests.yml

Progress of the PR

• [x] Change implemented (can be split into several points),
• [n/a] docstring updated (if appropriate),
• [n/a] update user guide (if appropriate),
• [x] added tests,
• [x] added line to CHANGELOG.md,
• [x] ready for review.

Minimal example of the bug fix or the new feature

import hyperspy.api as hs
S = hs.signals.Signal1D(np.arange(100),axes=[{'axis': np.arange(100)+300}])
S.set_signal_type("CL")
S.to_eV(inplace=True)
S.isig[3.251:4.052]

[codecov-commenter]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage :thumbsup:

Coverage data is based on head (d7a90ca) compared to base (58ade09). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #156 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 11 11 Lines 550 550 ========================================= Hits 550 550 ``` | [Impacted Files](https://codecov.io/gh/LumiSpy/lumispy/pull/156?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LumiSpy) | Coverage Δ | | |---|---|---| | [lumispy/utils/axes.py](https://codecov.io/gh/LumiSpy/lumispy/pull/156/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LumiSpy#diff-bHVtaXNweS91dGlscy9heGVzLnB5) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LumiSpy). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LumiSpy)

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[jlaehne]

@mikelgr11 you had observed the problem reported in #155, which should fixed with this PR.