Headerparser

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- Include `py.typed` file in distribution

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- Support Python 3.8 through 3.12
- Drop support for Python 2.7, 3.4, 3.5, and 3.6
- Removed `scan_file()`, `scan_lines()`, `HeaderParser.parse_file()`, and
`HeaderParser.parse_lines()` (all deprecated in v0.4.0)
- Type annotations added
- The scanner options to the scanner functions are now keyword-only
- `scan()` and `scan_stanzas()` can now parse strings directly. As a result,
`scan_string()` and `scan_stanzas_string()` are now deprecated.
- The `HeaderParser` methods `parse()` and `parse_stanzas()` can now parse
strings directly. As a result, the `parse_string()` and
`parse_stanzas_string()` methods are now deprecated.
- Added a `Scanner` class with methods for scanning a shared input. As a
result, the following are now deprecated:
- `scan_next_stanza()`
- `scan_next_stanza_string()`
- `HeaderParser.parse_next_stanza()`
- `HeaderParser.parse_next_stanza_string()`

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- Added a `scan()` function combining the behavior of `scan_file()` and
`scan_lines()`, which are now deprecated
- Gave `HeaderParser` a `parse()` method combining the behavior of
`parse_file()` and `parse_lines()`, which are now deprecated
- Added `scan_next_stanza()` and `scan_next_stanza_string()` functions for
scanning & consuming input only up to the end of the first header section
- Added `scan_stanzas()` and `scan_stanzas_string()` functions for scanning
input composed entirely of multiple stanzas/header sections
- Gave `HeaderParser` `parse_next_stanza()` and `parse_next_stanza_string()`
methods for parsing & consuming input only up to the end of the first header
section
- Gave `HeaderParser` `parse_stanzas()` and `parse_stanzas_string()` methods
for parsing input composed entirely of multiple stanzas/header sections

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- Drop support for Python 3.3
- Gave `HeaderParser` and the scanner functions options for configuring
scanning behavior:
- `separator_regex`
- `skip_leading_newlines`
- Fixed a `DeprecationWarning` in Python 3.7

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- `NormalizedDict`'s default normalizer (exposed as the `lower()` function) now
passes non-strings through unchanged
- `HeaderParser` instances can now be compared for non-identity equality
- `HeaderParser.add_field()` and `HeaderParser.add_additional()` now take an
optional `action` argument for customizing the parser's behavior when a field
is encountered
- Made the `unfold()` function public

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Initial release