Deprecate __ALL__, use __all__ instead

[hugovk]

Re: hfeeki/termcolor#3

__ALL__ should be lowercase: https://docs.python.org/3/tutorial/modules.html#importing-from-a-package

Let's deprecate the old one and remove it in the next major bump.

An up-to-date __all__ is in __init__.py

---

Also convert a % formatting to f-string, and add module docstring.

[codecov]

Codecov Report

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

Coverage data is based on head (e8b320f) compared to base (81b6ed5).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #23   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines           69        77    +8     
=========================================
+ Hits            69        77    +8     

Flag	Coverage Δ
macos-latest	100.00% <100.00%> (ø)
ubuntu-latest	100.00% <100.00%> (ø)
windows-latest	100.00% <100.00%> (ø)

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Impacted Files	Coverage Δ
src/termcolor/__init__.py	100.00% <ø> (ø)
src/termcolor/termcolor.py	100.00% <100.00%> (ø)
tests/test_termcolor.py	100.00% <100.00%> (ø)

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