Bump lxml from 4.6.2 to 4.6.3 (wesm#136)

Bumps [lxml](https://github.com/lxml/lxml) from 4.6.2 to 4.6.3.
- [Release notes](https://github.com/lxml/lxml/releases)
- [Changelog](https://github.com/lxml/lxml/blob/master/CHANGES.txt)
- [Commits](lxml/lxml@lxml-4.6.2...lxml-4.6.3)

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements.txt

@@ -1,7 +1,7 @@
 numpy==1.13.3
 pandas==0.21.0
 matplotlib==2.1.0
-lxml==4.6.2
+lxml==4.6.3
 seaborn==0.8.1
 statsmodels==0.8.0
 scipy==1.0.0