Fix issue with NULL columns

[waltervos]

Summary

As reported in #68, the parser stopped on columns with an explicit NULL definition. With my limited understanding of this package, I've tried to create a fix. I've added some columns in the basic test case which all pass.

Added

• CaselessKeyword for NULL
• Tests for NULL and NOT NULL columns
• Added supports for Python 3.9

Changed

• Detection of NULL/NOT NULL constraint now implemented using an Or match without regular expression.
• Migrate Travis-CI domain.

Removed

• Regular expression for NOT NULL constraints

Fixed

• The parser stops when it encounters a NULL column definition #68: Parser stops when it encounters a NULL column definition

Applicable Issues

• fix MySQL CREATE TABLE parsing fails. some columns are missing. #61
• fix The parser stops when it encounters a NULL column definition #68

[codecov]

Codecov Report

Merging #70 (a5f6dea) into master (cce0a1e) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #70   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines          391       391           
=========================================
  Hits           391       391           

Impacted Files	Coverage Δ
ddlparse/ddlparse.py	100.00% <ø> (ø)
ddlparse/__init__.py	100.00% <100.00%> (ø)

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[coveralls]

Coverage remained the same at 100.0% when pulling a5f6dea on waltervos:master into cce0a1e on shinichi-takii:master.

[shinichi-takii]

@waltervos
Excellent work!!

I added a few fixes.

Thank you!!!