Add support for empty data type annotation in SchemaModel

[jeffzi]

Fixes #522.

example:

import pandera as pa

class Model(pa.SchemaModel):
    a: pa.typing.Series
    
assert pa.DataFrameSchema({"a": pa.Column()}) == Model.to_schema()

I also took the opportunity to remove artifacts from py3.6 support that was dropped recently.

[codecov]

Codecov Report

Merging #602 (5b5f63e) into dev (ec1e084) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #602      +/-   ##
==========================================
+ Coverage   98.78%   98.84%   +0.05%     
==========================================
  Files          29       29              
  Lines        3386     3380       -6     
==========================================
- Hits         3345     3341       -4     
+ Misses         41       39       -2     

Impacted Files	Coverage Δ
pandera/model.py	100.00% <100.00%> (+0.84%)	⬆️
pandera/typing.py	98.43% <100.00%> (-0.07%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ec1e084...5b5f63e. Read the comment docs.

[cosmicBboy]

docs build is failing for some unknown reason 🤷‍♂️

[cosmicBboy]

looks like it's downloading a bunch of frictionless versions... do you mind pinning it to the latest version >=4.16.6?

[jeffzi]

The root cause is a dependency of frictionless, simpleeval, failing to install because of a deprecation in the recent version 58.0.0 of setuptools. See simpleeval issue and setuptools

I attempted to pin setuptools but readthedocs seems to force the upgrade? One of the build step is /home/docs/checkouts/readthedocs.org/user_builds/pandera/envs/602/bin/python -m pip install --upgrade --no-cache-dir pip setuptools

[cosmicBboy]

thanks for looking into this! seems like it's enough of a problem that it'll get fixed soon. Since this isn't related to the PR let's merge this and wait for upstream issue to be fixed before next bugfix release.