chore(pin): templateflow ~= 0.6 (and remove requirements.txt)

Taking advantage of the new auto-update. This commit also removes the
requirements.txt file that was used exclusively by the Docker image
distribution: pyntcloud now is in Pypi and we can safely pin
templateflow to >=0.6.0 since the autoupdate was implemented.

Author: oesteban

docs/requirements.txt

@@ -10,4 +10,4 @@ sphinx-argparse
 sphinx>=2.1.2,<3.0
 sphinx_rtd_theme
 sphinxcontrib-apidoc ~= 0.3.0
-templateflow >= 0.6.0rc1
+templateflow

setup.cfg

@@ -40,7 +40,7 @@ install_requires =
     seaborn
     svgutils
     transforms3d
-    templateflow >= 0.6.0rc1
+    templateflow >= 0.6
 test_requires =
     coverage < 5
     pytest >= 4.4