Include tests in the source distribution (#424)

The repository already ships certifi/tests, but MANIFEST.in only
listed package data files, so sdist builds omitted the suite.
Add a recursive-include so downstream can test from the sdist.

Closes #226

Signed-off-by: Alex Chen <l46983284@gmail.com>

MANIFEST.in

diff --git a/MANIFEST.in b/MANIFEST.in
index 25be160..eb42bc4 100644
--- a/MANIFEST.in
+++ b/MANIFEST.in
@@ -1,4 +1,6 @@
 include MANIFEST.in README.rst LICENSE certifi/cacert.pem certifi/py.typed
+recursive-include certifi/tests *
 
 exclude .github/
 recursive-exclude .github
+global-exclude *.py[cod] __pycache__ .DS_Store