Bump lxml from 4.6.2 to 4.6.3 in /build (#55)

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] <[email protected]>

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

Author: dependabot[bot]

build/requirements.txt

@@ -1,3 +1,3 @@
 wordfreq==2.3.2
-lxml==4.6.2
+lxml==4.6.3
 chardet==4.0.0