Remove requirements that were only there for messytables

sandbox/requirements3.txt

@@ -13,20 +13,14 @@ backports.functools-lru-cache==1.6.4
 chardet==4.0.0
 enum34==1.1.10
 et-xmlfile==1.0.1
-html5lib==1.1
 iso8601==0.1.12
 jdcal==1.4.1
-json_table_schema==0.2.1
 lazy_object_proxy==1.6.0
-lxml==4.6.3                # used in csv plugin only?
 python_dateutil==2.8.2
 openpyxl==3.0.10
-python_magic==0.4.12
 roman==2.0.0
 singledispatch==3.6.2
 six==1.16.0
 sortedcontainers==2.4.0
-webencodings==0.5
 wrapt==1.12.1
-xlrd==1.2.0
 unittest-xml-reporting==2.0.0