Add support for Python raw docstrings

Sometimes, in Python, a docstring will be a raw (`r"..."`) string, as
opposed to a regular (`"..."`) string (I've seen this when a docstring
needs to escape things in funny ways for sphinx). This commit adds two
entries to the `Python.quote` section of `languages.json` (and the
corresponding update to `constants.go`) for recognizing these raw
docstrings as docs rather than code.

For example, without this commit, for the file

```python
r"""Welcome to foo.py! Sometimes I escape :class:`thing`\s funny when
Sphinx is involved.
"""

def hello():
    pass
```

`scc` reports

```
───────────────────────────────────────────────────────────────────────────────
Language                 Files     Lines   Blanks  Comments     Code Complexity
───────────────────────────────────────────────────────────────────────────────
Python                       1         6        0         0        6          0
───────────────────────────────────────────────────────────────────────────────
Total                        1         6        0         0        6          0
───────────────────────────────────────────────────────────────────────────────
Estimated Cost to Develop (organic) $125
Estimated Schedule Effort (organic) 0.45 months
Estimated People Required (organic) 0.02
───────────────────────────────────────────────────────────────────────────────
Processed 117 bytes, 0.000 megabytes (SI)
───────────────────────────────────────────────────────────────────────────────
```

But with this commit, reports

```
───────────────────────────────────────────────────────────────────────────────
Language                 Files     Lines   Blanks  Comments     Code Complexity
───────────────────────────────────────────────────────────────────────────────
Python                       1         6        1         3        2          0
───────────────────────────────────────────────────────────────────────────────
Total                        1         6        1         3        2          0
───────────────────────────────────────────────────────────────────────────────
Estimated Cost to Develop (organic) $39
Estimated Schedule Effort (organic) 0.29 months
Estimated People Required (organic) 0.01
───────────────────────────────────────────────────────────────────────────────
Processed 117 bytes, 0.000 megabytes (SI)
───────────────────────────────────────────────────────────────────────────────
```

Thanks for your consideration!

languages.json

@@ -5360,6 +5360,16 @@
         "docString": true,
         "end": "'''",
         "start": "'''"
+      },
+      {
+        "docString": true,
+        "end": "\"\"\"",
+        "start": "r\"\"\""
+      },
+      {
+        "docString": true,
+        "end": "'''",
+        "start": "r'''"
       }
     ],
     "shebangs": [