Use tomlkit for TOML read/write operations

[dataflake]

This PR is an alternative to #209. It does two things:

• replace toml use with tomlkit
• update dependencies in requirements.txt to the latest versions that still support Python 3.7

Running this code against current Zope yields the following diff in .meta.toml:

diff --git a/.meta.toml b/.meta.toml
index 82f0c085f..07cac2b24 100644
--- a/.meta.toml
+++ b/.meta.toml
@@ -2,7 +2,7 @@
 # https://github.com/zopefoundation/meta/tree/master/config/zope-product
 [meta]
 template = "zope-product"
-commit-id = "571024f0"
+commit-id = "babc889c"

 [python]
 with-pypy = false
@@ -11,6 +11,7 @@ with-sphinx-doctests = false
 with-windows = true
 with-macos = true
 with-future-python = true
+with-appveyor = false

 [coverage]
 fail-under = 80

The different is most likely due to the change in the _set_python_config_value method where I always use the default False because some values were None at that point, which tomlkit didn't like.

[dataflake]

Instead of changing the behavior of _set_python_config_value I have decided to fix the real reason: Several script arguments that want to store true or false had an inconsistent default of None. I have corrected those. The outcome is still the same as the diff shown above.

[sallner]

We can also do it this way, but it actually does not use the capabilities of tomlkit to add and preserve comments now. It does the same as before, so I am fine from this perspective.

[sallner]

It looks okay to me, although we might end up with all default values in the meta.toml now.

[dataflake]

It looks okay to me, although we might end up with all default values in the meta.toml now.

I was thinking about that and to be honest I don't think that's a bad thing. It's very explicit.

[icemac]

Finally I got to this PR.

[icemac]

Thank you for this PR. 😃