Fix docs dependencies spec, resolve docs version automatically (#180)

Proof it works:

```py
>>> from docs import conf

>>> conf.version
'2.1'

>>> conf.release
'2.1.0'
```

Author: snejus

.readthedocs.yaml

@@ -12,3 +12,5 @@ python:
   install:
     - method: pip
       path: .
+      extra_requirements:
+        - docs

docs/conf.py

@@ -1,33 +1,37 @@
+import datetime as dt
 import os
+import re
 import sys
-import datetime as dt
+from pathlib import Path
+
+MATCH_VERSION_LINE = re.compile(r"version = \W((\d+\.\d+)\.\d.*?)\W").fullmatch
 
+pyproject = Path(__file__).parent.parent / "pyproject.toml"
+version_line_match = next(
+    filter(None, map(MATCH_VERSION_LINE, pyproject.read_text().splitlines()))
+)
+release, version = version_line_match.groups()
 
 sys.path.insert(0, os.path.abspath(".."))
 
 extensions = [
-    'sphinx.ext.autodoc',
-    'sphinx.ext.intersphinx',
-    'sphinx.ext.viewcode',
-    'sphinx.ext.autosectionlabel',
-    'sphinx_rtd_theme',
+    "sphinx.ext.autodoc",
+    "sphinx.ext.intersphinx",
+    "sphinx.ext.viewcode",
+    "sphinx.ext.autosectionlabel",
+    "sphinx_rtd_theme",
 ]
-source_suffix = '.rst'
-master_doc = 'index'
-
-project = u'Confuse'
-copyright = '2012-{}, Adrian Sampson & contributors'.format(
-    dt.date.today().year
-)
+source_suffix = ".rst"
+master_doc = "index"
 
-version = '1.7'
-release = '1.7.0'
+project = "Confuse"
+copyright = "2012-{}, Adrian Sampson & contributors".format(dt.date.today().year)
 
-exclude_patterns = ['_build']
+exclude_patterns = ["_build"]
 
-pygments_style = 'sphinx'
+pygments_style = "sphinx"
 
 # -- Options for HTML output --------------------------------------------------
 
-html_theme = 'sphinx_rtd_theme'
-htmlhelp_basename = 'Confusedoc'
+html_theme = "sphinx_rtd_theme"
+htmlhelp_basename = "Confusedoc"