Add py.typed marker for PEP 561 compatibility (#657)

Via PR by @jhonatan-lopes.

CHANGELOG.md

@@ -10,6 +10,7 @@ All notable changes to this project will be documented in this file. The format
 - Add `pdfplumber.ctm` submodule with class `CTM`, to calculate scale, skew, and translation of a current transformation matrix obtained from a `char`'s `"matrix"` property.
 - Add `page.search(...)`, an *experimental feature* that allows you to search a page's text via regular expressions and non-regex strings, returning the text, any regex matches, the bounding box coordinates, and the char objects themselves. ([#201](https://github.com/jsvine/pdfplumber/issues/201))
 - Add `--include-attrs`/`--exclude-attrs` to CLI (and corresponding params to `.to_json(...)`, `.to_csv(...)`, and `Serializer`.
+- Add `py.typed` for PEP561 compatibility and detection of typing hints by mypy.
 
 ### Removed
 

MANIFEST.in

@@ -1,2 +1,3 @@
 include LICENSE.txt
 include README.md
+include pdfplumber/py.typed