Revert "CI: pin pytables to valid build (pandas-dev#17760)" (pandas-dev#17768)

This reverts commit 6d30d5f.

Author: jreback

ci/requirements-3.6.run

@@ -7,7 +7,7 @@ xlsxwriter
 xlrd
 xlwt
 numexpr
-pytables=3.4.2=np113py36_1
+pytables
 matplotlib
 lxml
 html5lib