Adds pyenv step

[lucaspar]

• Closes Support for pyenv #626.

Runs pyenv update as the new pyenv step, given that all conditions are met:

• OS is Unix (Windows not supported by Pyenv);
• The $PYENV_ROOT env variable points to a valid location; and
• $PYENV_ROOT points to a Git directory.
  • This last check might not be the case for other methods of installing pyenv, for which a pyenv update may not be available, or may fail.

Standards checklist:

• The PR title is descriptive.
• I have read CONTRIBUTING.md
• The code compiles (cargo build)
• The code passes rustfmt (cargo fmt)
• The code passes clippy (cargo clippy)
• The code passes tests (cargo test)
• Optional: I have tested the code myself
  • I ran cargo run -- --verbose --only pyenv, which works as expected for me, but I'm not sure where/how to add automated unit tests.

For new steps

• Optional: Topgrade skips this step where needed
• Optional: The --dry-run option works with this step
• Optional: The --yes option works with this step if it is supported by
  the underlying command
  • underlying command does not support --yes.

If you developed a feature or a bug fix for someone else and you do not have the
means to test it, please tag this person here.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 21 lines in your changes are missing coverage. Please review.

Project coverage is 5.57%. Comparing base (9b6027f) to head (ee2eeeb).

Files	Patch %	Lines
src/steps/os/unix.rs	0.00%	19 Missing ⚠️
src/config.rs	0.00%	1 Missing ⚠️
src/main.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##            main    #724      +/-   ##
========================================
- Coverage   5.65%   5.57%   -0.09%     
========================================
  Files         37      37              
  Lines      11503   11675     +172     
========================================
  Hits         651     651              
- Misses     10852   11024     +172     

☔ View full report in Codecov by Sentry.
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[SteveLauC]

Don't worry about the codecov CI failures, they don't matter for now.

[SteveLauC]

Even though the pyenv command does not support the --dry-run option, Topgrade will handle it for you:

$ cargo r -- --only pyenv --dry-run

[SteveLauC]

Thanks!