improve maintainability index mi 55.888 -> 91.1844

[ronnydw]

Describe your change:

This is a POC to demonstrate measuring and improving maintainability (#2128 (comment))

Measured with wily:
Ronnys-MacBook-Pro:Python ronny$ wily diff strings/lower.py
Using default metrics ['maintainability.mi', 'halstead.h1', 'cyclomatic.complexity', 'raw.loc']
Comparing current with b934da4 by Dhruv on 2020-10-04.

╒═══════════════════════════════════════╤═════════════════════════╤═══════════════════╤═════════════════════════╤═════════════════╕
│ File                                  │ Maintainability Index   │ Unique Operands   │ Cyclomatic Complexity   │ Lines of Code   │
╞═══════════════════════════════════════╪═════════════════════════╪═══════════════════╪═════════════════════════╪═════════════════╡
│ strings/lower.py                      │ 55.888 -> 91.1844       │ 3 -> 3            │ 3 -> 7                  │ 26 -> 30        │
├───────────────────────────────────────┼─────────────────────────┼───────────────────┼─────────────────────────┼─────────────────┤

The following changes improved the maintainability index from 55.888 to 91.1844:

• extracted method for chr(ord(capital) + 32)
• replace if operator with 2x '<=' by a const CAPITALS and 'in' operator
• rename 'word' to 'string'
• remove redundant comments
• replace doctest comments by real test code

For manacher.py:

• fix bug: should return '' for input ''
• add some more tests
• type-hinting
• better naming
• ref to wikipedia
• maintainability 59.4401 -> 74.4849

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