Update cpython requirement from 0.4.1 to 0.5.0

[dependabot-preview[bot]]

Updates the requirements on cpython to permit the latest version.

Changelog

Sourced from cpython's changelog.

0.5.0 - 2020-04-08

• [properties (attributes with getter/setters defined in Rust][208] (PR by [@markbt])
• [adoption of 2018 edition and general code modernization][204] (PR by [@markbt])
• [reference extraction for slot functions and optional reference extraction][207] (PR by [@markbt])
• [PEP-587 initialization APIs (python3-sys for Python≥3.8)][211] (PR by [@indygreg])
• [more import APIs (python3-sys)][210] (PR by [@indygreg])

0.4.1 - 2020-02-03

• link-time inconsistency with build config (original PR by @svevang adapted as 202)
• missing pub classifier in PySharedRef example. (PR by @Alphare)
• README updates: copyright years, version number in examples

0.4.0 - 2020-01-27

• The 0.4.x series is planned to be the last that will support Rust 2015.
• Added Python 3.8 support.
• Type errors during downcasts now explain what the expected and actual types are. (PR by [@markbt])
• Data items can now be shared between Python objects (e.g. for iterators) using PySharedRef. (PR by @yuja)
• PyList can now be appended to with append. The method for inserting items is renamed to insert for consistency. (PR by @binh-vu)

0.3.0 - 2019-08-01

• Minumum Rust compiler version is now 1.30.
• Added Capsule support (PR by @gracinet)
• Added Rust Docstring support for instance methods, [static methods and class methods][179] (PRs by @quark-zju and [@AdamRzepka])
• Made macros work with Rust 2018 (PR by @derekdreery)
• Support Rust raw identifiers for method and keyword names (PR by @quark-zju)
• Added impl RefFromPyObject for [u8]. This allows using &[u8] as parameter type in py_fn!. When passing a bytes object from Python, this allows accessing the data without a copy. (other mutable Python objects will use a defensive copy)

... (truncated)

Commits

• 690ac16 Version 0.5.0
• 9b9a734 Properties doc: about deletion
• 1c80d4a properties doc: clarification vs Python properties
• 387e87d Merge pull request #211 from indygreg/initialization-apis
• 599f2fb Implement PEP-587 initialization APIs
• 7fb4dd2 Merge pull request #208 from markbt/properties
• 77393d5 Merge pull request #207 from markbt/refslots
• 76fbba6 Merge pull request #210 from indygreg/more-import-apis
• 8d2f857 Add more import APIs
• 64cab6a handle properties with raw identifier names
• Additional commits viewable in compare view

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[codecov-io]

Codecov Report

Merging #25 into master will increase coverage by 5.12%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #25      +/-   ##
==========================================
+ Coverage   79.21%   84.33%   +5.12%     
==========================================
  Files           1        1              
  Lines         279      249      -30     
==========================================
- Hits          221      210      -11     
+ Misses         58       39      -19     

Impacted Files	Coverage Δ
src/lib.rs	84.33% <0.00%> (+5.12%)	:arrow_up:

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