Parse style from <style> tag and Add support to <line> tag

[TonyCrane]

Motivation

#1717 (comment)
I solved this issue. Use tinycss2 to parse style (including <style> tag and style attributes in other tags), and use cssselect2 to choose which tag to apply to based on selector.
And in order to make cssselect2 handle selectors correctly, I replaced xml.dom.minidom with xml.etree for parsing SVG file, which also has more features.

At the same time, I also implemented the processing of the <line> tag by the way.

Proposed changes

• M manimlib/mobject/svg/svg_mobject.py: improve style parsing and implement the <line> tag
• M setup.cfg & requirements.txt: add dependency of cssselect2(tinycss2 will be installed along with it)

Test

Code:

class Code(Scene):
    def construct(self):
        svg = SVGMobject("beer.svg", color=BLACK, height=6)
        self.add(svg)

Result:

Code:

class Code(Scene):
    def construct(self):
        svg = SVGMobject("line.svg")
        self.add(svg)

Result: