[BUG] SVG rendering returns bytes object, leading to XML Parsing Error
**Describe the bug**
When attempting to render an SVG file, I get an "XML Parsing Error: not well-formed [...] Line Number 1, Column 2:" in Firefox.

```
b'<?xml version="1.0" encoding="UTF-8"?>\\n ...
```
Note the b' in the beginning, it looks like a string representation of a bytes object is returned

**To Reproduce**
Enter the rendering URL https://example.com/render?&target=test.count&format=svg

**Expected behavior**
The requests succeeds with a valid SVG file.

**Screenshots**
If applicable, add screenshots to help explain your problem.

**Environment (please complete the following information):**
 - Debian Buster
 - Graphite-web version 1.1.6
 - Python 3.7.3
 - Django version 2.2.7
 - graphite-web and Django installed via pip from pypi (pip install graphite-web) in a venv
 - run/started via uwsgi (also installed in venv), served by nginx

**Additional context**

uwsgi ini file:
```
[uwsgi]
processes = 1
socket = localhost:12222
virtualenv = /opt/graphite
chdir = /opt/graphite/conf
module = wsgi:application
```

```
# conf/wsgi.py 
import sys
import os

sys.path.append('/opt/graphite/webapp')

from graphite.wsgi import application
```
diff --git a/webapp/graphite/render/glyph.py b/webapp/graphite/render/glyph.py
index 2851321a..9d999426 100644
--- a/webapp/graphite/render/glyph.py
+++ b/webapp/graphite/render/glyph.py
@@ -885,10 +885,10 @@ class Graph:
         metaData = { }
 
       self.surface.finish()
-      svgData = str(self.surfaceData.getvalue())
+      svgData = self.surfaceData.getvalue().decode('utf-8')
       self.surfaceData.close()
 
-      svgData = svgData.replace('pt"', 'px"', 2) # we expect height/width in pixels, not points
+      svgData = svgData.replace('pt"', 'px"', 2)  # we expect height/width in pixels, not points
       svgData = svgData.replace('</svg>\\n', '', 1)
       svgData = svgData.replace('</defs>\\n<g', '</defs>\\n<g class="graphite"', 1)
 
@@ -903,10 +903,10 @@ class Graph:
         (svgData, subsMade) = re.subn(r'<path.+?d="M -88 -88 (.+?)"/>', onHeaderPath, svgData)
 
         # Replace the first </g><g> with <g>, and close out the last </g> at the end
-        svgData = svgData.replace('</g><g data-header','<g data-header',1)
+        svgData = svgData.replace('</g><g data-header', '<g data-header', 1)
         if subsMade > 0:
-          svgData += "</g>"
-        svgData = svgData.replace(' data-header="true"','')
+          svgData += '</g>'
+        svgData = svgData.replace(' data-header="true"', '')
 
       fileObj.write(svgData.encode('utf-8'))
       fileObj.write(("""<script>
['webapp/graphite/render/glyph.py']