setup.py: do not overwrite conda's PyQt5

The PyQt5 dependency conflicts with conda, where the same package is called
pyqt. Installing pyqt5 inside a conda environment that already contains pyqt
may be catastrophic. Removing the dependency makes pip installation into conda
environments safer; such installations frequently occur when users install an
add-on not available in the conda repos that requires a newer version of
Orange. The same issue appears when Anaconda users try to update Orange with
the Add-on dialog box. Or when someone who installed Orange with conda
downloads the master branch and does "pip install -e .".

Orange added the PyQt5 dependency about a year ago to make pip installations
friendlier. Unfortunately, that caused more problems than it fixed. This commit
modifies setup.py to only installs PyQt5 in a conda environment if PyQt5 could
not be imported. I presume this is safe.

requirements-gui.txt

@@ -1,8 +1,6 @@
 orange-canvas-core>=0.1.21,<0.2a
 orange-widget-base>=4.13.0
 
-PyQt5>=5.12,!=5.15.1  # 5.15.1 skipped because of QTBUG-87057 - affects select columns
-PyQtWebEngine>=5.12
 AnyQt>=0.0.11
 
 pyqtgraph>=0.11.1

requirements-pyqt.txt

@@ -0,0 +1,2 @@
+PyQt5>=5.12,!=5.15.1  # 5.15.1 skipped because of QTBUG-87057 - affects select columns
+PyQtWebEngine>=5.12

setup.py

@@ -34,6 +34,13 @@
 except ImportError:
     have_cython = False
 
+try:
+    import PyQt5.QtCore  # pylint: disable=unused-import
+    have_pyqt5 = True
+except ImportError:
+    have_pyqt5 = False
+
+is_conda = os.path.exists(os.path.join(sys.prefix, 'conda-meta'))
 
 NAME = 'Orange3'
 
@@ -78,6 +85,12 @@
 
 requirements = ['requirements-core.txt', 'requirements-gui.txt']
 
+# pyqt5 is named pyqt5 on pypi and pyqt on conda
+# due to possible conflicts, skip the pyqt5 requirement in conda environments
+# that already have pyqt
+if not (is_conda and have_pyqt5):
+    requirements.append('requirements-pyqt.txt')
+
 INSTALL_REQUIRES = sorted(set(
     line.partition('#')[0].strip()
     for file in (os.path.join(os.path.dirname(__file__), file)