Enable selection of quoting style during etree.tostring

[Stdedos]

From https://stackoverflow.com/questions/46566216/writing-lxml-etree-with-double-quotes-header-attributes:

======================
I have created a basic xml tree using lxml tutorial:

```
from lxml import etree
root = etree.Element("root")
root.append( etree.Element("child1") )
child2 = etree.SubElement(root, "child2")
child3 = etree.SubElement(root, "child3")
print(etree.tostring(root, pretty_print=True, encoding="UTF-8", xml_declaration=True))
```
This produces the following:

```
<?xml version='1.0' encoding='UTF-8'?>
<root>
  <child1/>
  <child2/>
  <child3/>
</root>
```
My question is, how to produce xml file with double quoted file header, i.e.

```
<?xml version="1.0" encoding="UTF-8"?>
```
======================

The specific issue would be (master at-the-time: 68cf93c4827ea74e46d2aa6809011f96ed9c689a)
https://github.com/lxml/lxml/blob/68cf93c4827ea74e46d2aa6809011f96ed9c689a/src/lxml/serializer.pxi#L276-L285

However, it would be nice if every other instance of "quote style" would be configurable too, if any.

---------
## -- PLEASE PROVIDE THE FOLLOWING INFORMATION: --
Python : sys.version_info(major=3, minor=5, micro=2, releaselevel='final', serial=0)
lxml.etree : (4, 1, 1, 0)
libxml used : (2, 9, 7)
libxml compiled : (2, 9, 7)
libxslt used : (1, 1, 32)
libxslt compiled : (1, 1, 32)
## -----

[scoder (scoder)]

As the SO discussion suggests, the output is perfectly correct. It's the "quality gate" that needs fixing.

Even if there was a way in lxml to change the quoting, it would only apply to a few parts of the output, since most of the serialisation is done by libxml2, which does not have a way to configure it.

Writing your own declaration into a file before serialising the rest of the XML seems like a good enough work-around, though, that gives complete control over the spelling. I don't think there is anything to improve here.