Modified test for numeric types.

Update changes.rst for release 1.7.4

- Added python 3.9 for build on travis-ci
- Put python 2.7 bellow in the matrix list for avoiding failure while running deploy on a release tag v#.#.#
- travis-ci will only trigger on main repo github.com/petl-developers/petl
- Skipped doctest with python3.9 for now

Update changes.rst for release 1.7.3

Updated changes for release 1.7.2

conda: fixing packaging failures on feedstock

update changelog for v1.7.0

more fixes for xml test failure in ci

Changes Written in docs/*.rst

relax more deps for py38

updated changes for v1.6.5