[Fix]: fix mim command crashes problem if requirements version conflict

[ice-tong]

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Motivation

Fixes #143 that mim could be crash if exist version conflict.
For reproduce mim crash:

> pip install markdown==3.3.7
> pip install importlib-metadata==4.2
> mim list

Modification

• mim/__init__.py: set __requires__ in __main__ module from 'openmim' to '' avoid requirement resolve check.

BC-breaking (Optional)

Does the modification introduce changes that break the back-compatibility of the downstream repos?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMCls.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[codecov]

Codecov Report

Merging #144 (c8703b7) into main (b449e97) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #144      +/-   ##
==========================================
+ Coverage   54.44%   54.50%   +0.05%     
==========================================
  Files          23       23              
  Lines        1552     1554       +2     
  Branches      331      331              
==========================================
+ Hits          845      847       +2     
  Misses        582      582              
  Partials      125      125              

Flag	Coverage Δ
unittests	54.50% <100.00%> (+0.05%)	⬆️

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Impacted Files	Coverage Δ
mim/__init__.py	100.00% <100.00%> (ø)

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[ice-tong]

This workaround works fine, but I'm not sure if good enough to merge into the master branch. Please give this PR a review. @zhouzaida