🐛 Move source into src/ to fix install

Installing was broken since moving to pyproject.toml, which we didn't notice because of
leftover files in build/. Fix this by using the convention of having the source files
in src/ and adjusting pyproject.toml accordingly.

Fixes gh-86. 🤞

src/dinglehopper/tests/test_edit_distance.py

@@ -0,0 +1,24 @@
+from __future__ import division, print_function
+
+import unicodedata
+
+from .. import distance
+
+
+def test_distance():
+    assert distance("Fnord", "Food") == 2
+    assert distance("Müll", "Mull") == 1
+
+    word1 = unicodedata.normalize("NFC", "Schlyñ")
+    word2 = unicodedata.normalize("NFD", "Schlyñ")  # Different, decomposed!
+    assert distance(word1, word2) == 0
+
+    word1 = "Schlyñ"
+    assert (
+        len(word1) == 6
+    )  # This ends with LATIN SMALL LETTER N WITH TILDE, so 6 code points
+    word2 = "Schlym̃"
+    assert (
+        len(word2) == 7
+    )  # This, OTOH, ends with LATIN SMALL LETTER M + COMBINING TILDE, 7 code points
+    assert distance(word1, word2) == 1