Handle ASCII help output streams

[puneetdixit200]

Fixes #1411.

Summary:

• Fall back to ASCII-safe escape sequences when argparse help text is written to a strict ASCII stream.
• Add a regression test for http --help with ASCII-only stdout.

Tests:

• python -m pytest tests/test_httpie.py::test_help tests/test_httpie.py::test_help_with_ascii_stdout -q
• python -m pytest tests/test_httpie.py -q
• python -m pytest tests/test_cli.py -q
• python -m pytest tests/test_cli_utils.py -q
• python -m flake8 httpie/cli/argparser.py tests/test_httpie.py
• git diff --check

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 85.71429% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.10%. Comparing base (4d7d6b6) to head (e3ac637).
⚠️ Report is 383 commits behind head on master.

Files with missing lines	Patch %	Lines
httpie/cli/argparser.py	70.00%	3 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1852      +/-   ##
==========================================
- Coverage   97.28%   94.10%   -3.19%     
==========================================
  Files          67      113      +46     
  Lines        4235     7712    +3477     
==========================================
+ Hits         4120     7257    +3137     
- Misses        115      455     +340     

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